World War II, also WWII, or the Second World War, was a global military conflict that took place between 1939 and 1945. It was the largest and deadliest war in history. The date commonly given for the start of the war is September 1, 1939, when Nazi Germany invaded Poland. Within two days the United Kingdom and France declared war on Germany, although the only European battles remained in Poland. Pursuant to a then-secret provision of its non-aggression Molotov-Ribbentrop Pact, the Soviet Union joined with Germany on September 17, 1939, to conquer Poland and to divide Eastern Europe.

The Allies were initially made up of Poland, the British Empire, France, and others. In May, 1940, Germany invaded western Europe. Six weeks later, France surrendered to Germany. Three months after that, Germany, Italy, and Japan signed a mutual defense agreement, the Tripartite Pact, and were known as the Axis Powers. Then, nine months later, in June 1941, Germany betrayed and invaded the Soviet Union, forcing the Soviets into the Allied camp (although they continued their non-aggression treaty with Japan). In December 1941, Japan attacked the United States bringing it into the war on the Allied side. China also joined the Allies, as eventually did most of the rest of the world. From the beginning of 1942 through August 1945, battles raged across all of Europe, in the North Atlantic Ocean, across North Africa, throughout Southeast Asia, and China, across the Pacific Ocean and in the air over Germany and Japan.

After World War II, Europe was split into western and Soviet spheres of influence. Western Europe later aligned as NATO and Eastern Europe as the Warsaw Pact. There was a shift in power from Western Europe and the British Empire to the two post-war superpowers, the United States and the Soviet Union. These two rivals would later face off in the Cold War. In Asia, Japan's defeat led to its democratization. China's civil war continued into the 1950s, resulting eventually in the establishment of the People's Republic of China. European colonies began their road to independence. Disgusted at the human cost of war, as people had been after World War I, a commitment to diplomacy to resolve differences was written into the charter of the new international body that replaced the failed League of Nations, the United Nations, which this time attracted U.S. support. The real effectiveness of this body has been subsequently compromised because member states act when it suits them, and sometimes by-pass it altogether. The victory, though, of the Allies over the Axis powers is usually regarded as having safeguarded democracy and freedom. The Holocaust represented one of the the most evil incidents in human history. Even still, the Allies cannot be said to have conducted the war according to the highest standards of combat, using mass bombings that provoked one leading British Bishop, George Bell (1883-1958) to withdraw his support for the just cause of the war.


Commonly held general causes for WWII are the rise of nationalism, militarism, and unresolved territorial issues. In Germany, resentment of the harsh Treaty of Versailles-specifically article 231 (the "Guilt Clause"), the belief in the Dolchstosslegende (that treachery had cost them WWI), and the onset of the Great Depression-fueled the rise to power of Adolf Hitler's militarist National Socialist German Workers Party (the Nazi Party). Meanwhile, the treaty's provisions were laxly enforced due to fear of another war. Closely related is the failure of the British and French policy of appeasement, which sought to avoid war but actually gave Hitler time to re-arm. The League of Nations proved to be ineffective.

Japan, ruled by a militarist clique devoted to becoming a world power invaded China to bolster its meager stock of natural resources. This angered the United States, which reacted by making loans to China, providing covert military assistance, and instituting increasingly broad embargoes of raw materials against Japan. These embargoes would have eventually wrecked Japan's economy; Japan was faced with the choice of withdrawing from China or going to war in order to conquer the oil resources of the Dutch East Indies (Indonesia). It chose to proceed with plans for the Greater East Asia War in the Pacific.

War breaks out in Europe: 1939

Pre-war alliances

In March 1939, when German armies entered Prague then occupied the remainder of Czechoslovakia, the Munich Agreement-which required Germany to peacefully resolve its claim to the Czech territory-collapsed. On May 19, Poland and France pledged to provide each other with military assistance in the event either was attacked. The British had already offered support to the Poles in March; then, on August 23, Germany and the Soviet Union signed the Molotov-Ribbentrop Pact. The pact included a secret protocol which would divide eastern Europe into German and Soviet areas of interest. Each country agreed to allow the other a free hand in its area of influence, including military occupation. Hitler was now ready to go to war in order to conquer Poland. The signing of a new alliance between Britain and Poland on August 25, deterred him for only a few days.

Invasion of Poland
A flight of Junkers Ju 87 -Stuka dive-bombers prepares to attack.

On September 1, Germany invaded Poland. Two days later, Britain and France declared war on Germany. The French mobilized slowly, then mounted a token offensive in the Saar, which they soon abandoned, while the British could not take any direct action in support of the Poles in the time available. Meanwhile, on September 9, the Germans reached Warsaw, having slashed through the Polish defenses.

On September 17, Soviet troops occupied the eastern Poland, taking control of territory that Germany had agreed was in the Soviet sphere of influence. A day later the Polish president and commander-in-chief both fled to Romania. The last Polish units surrendered on October 6. Some Polish troops evacuated to neighboring countries. In the aftermath of the September Campaign, occupied Poland managed to create a powerful resistance movement and Poles made a significant contribution to the Allies' cause for the duration of World War II.

After Poland fell, Germany paused to regroup during the winter of 1939-1940 until April 1940, while the British and French stayed on the defensive. The period was referred to by journalists as "the Phony War," or the "Sitzkrieg," because so little ground combat took place.

Battle of the Atlantic

Meanwhile in the North Atlantic, German U-boats operated against Allied shipping. The submarines made up in skill, luck, and daring what they lacked in numbers. One U-boat sank the British aircraft carrier HMS Courageous, while another managed to sink the battleship HMS Royal Oak in its home anchorage of Scapa Flow. Altogether, U-boats sank more than 110 vessels in the first four months of the war.

In the South Atlantic, the German pocket battleship Admiral Graf Spee raided Allied shipping, then was scuttled after the battle of the River Plate. About a year and a half later, another German raider, the battleship Bismarck, suffered a similar fate in the North Atlantic. Unlike the U-boat threat, which had a serious impact later in the war, German surface raiders had little impact because their numbers were so small.

War spreads: 1940

Soviet-Finnish War

The Soviet Union attacked Finland on November 30, 1939, starting the Winter War. Finland surrendered to the Soviet Union in March 1940 and signed the Moscow Peace Treaty (1940) in which the Finns made territorial concessions. Later that year, in June the Soviet Union occupied Latvia, Lithuania, and Estonia, and annexed Bessarabia and Northern Bukovina from Romania.

Invasion of Denmark and Norway
The evolution of German plans for the invasion of France.

Germany invaded Denmark and Norway on April 9, 1940, in part to counter the threat of an impending Allied invasion of Norway. Denmark did not resist, but Norway fought back, assisted by British, French, and Polish (exile) forces landing in support of the Norwegians at Namsos, Åndalsnes, and Narvik. By late June, the Allies were defeated, German forces were in control of most of Norway, and what remained of the Norwegian Army had surrendered.

Invasion of France and the Low Countries
Heinkel He 111 over London on 7 September 1940

On May 10, 1940, the Germans invaded Luxembourg, Belgium, the Netherlands, and France, ending the "Phony War." The British Expeditionary Force (BEF) and the French Army advanced into northern Belgium, planning to fight a mobile war in the north while maintaining a static continuous front along the Maginot Line, built after World War I, further south.

In the first phase of the invasion, Fall Gelb (CACA), the Wehrmacht's Panzergruppe von Kleist raced through the Ardennes, broke the French line at Sedan, then slashed across northern France to the English Channel, splitting the Allies in two. Meanwhile Belgium, Luxembourg, and the Netherlands fell quickly against the attack of German Army Group B. The BEF, encircled in the north, was evacuated from Dunkirk in Operation Dynamo. On June 10, Italy joined the war, attacking France in the south. German forces then continued the conquest of France with Fall Rot (Case Red), advancing behind the Maginot Line and near the coast. France signed an armistice with Germany on June 22, 1940, leading to the establishment of the Vichy France puppet government in the unoccupied part of France.

Battle of Britain

Following the defeat of France, Britain chose to fight on, so Germany began preparations in summer of 1940 to invade Britain (Operation Sea Lion), while Britain made anti-invasion preparations. Germany's initial goal was to gain air control over Britain by defeating the Royal Air Force (RAF). The war between the two air forces became known as the Battle of Britain. The Luftwaffe initially targeted RAF Fighter Command. The results were not as expected, so the Luftwaffe later turned to terror bombing London. The Germans failed to defeat the Royal Air Force, thus Operation Sea Lion was postponed and eventually canceled.

North African Campaign
Afrika Korps tanks advance during the North African campaign.

Italy declared war in June 1940, which challenged British supremacy of the Mediterranean, hinged on Gibraltar, Malta, and Alexandria. Italian troops invaded and captured British Somaliland in August. In September, the North African Campaign began when Italian forces in Libya attacked British forces in Egypt. The aim was to make Egypt an Italian possession, especially the vital Suez Canal east of Egypt. British, Indian, and Australian forces counter-attacked in Operation Compass, but this offensive stopped in 1941 when much of the Commonwealth forces were transferred to Greece to defend it from German attack. However, German forces (known later as the Afrika Korps) under General Erwin Rommel landed in Libya and renewed the assault on Egypt.

Invasion of Greece

Italy invaded Greece on October 28, 1940, from bases in Albania after the Greek Premier John Metaxas rejected an ultimatum to hand over Greek territory. Despite the enormous superiority of the Italian forces, the Greek army forced the Italians into a massive retreat deep into Albania. By mid-December, the Greeks occupied one-fourth of Albania. The Greek army had inflicted upon the Axis Powers their first defeat in the war, and Nazi Germany would soon be forced to intervene.

War becomes global: 1941

The extent of the Axis conquests during World War II

European theater


U.S. President Franklin Delano Roosevelt signed the Lend-Lease Act on March 11. This program was the first large step away from American isolationism, providing for substantial assistance to the United Kingdom, the Soviet Union, and other countries.

Invasion of Greece and Yugoslavia
German advances during Operation Barbarossa from June 1941 to December 1941.

Yugoslavia's government succumbed to the pressure of the Axis and signed the Tripartite Treaty on March 25, but the government was overthrown in a coup which replaced it with a pro-Allied government. This prompted the Germans to invade Yugoslavia on April 6. In the early morning, Germans bombarded Belgrade with an estimated 450 aircraft. Yugoslavia was occupied in a matter of days, and the army surrendered on April 17, but the partisan resistance lasted throughout the war. The rapid downfall of Yugoslavia, however, allowed German forces to enter Greek territory through the Yugoslav frontier. The 58,000 British and Commonwealth troops who had been sent to help the Greeks were driven back and soon forced to evacuate. On April 27, German forces entered Athens which was followed by the end of organized Greek resistance. The occupation of Greece proved costly, as guerilla warfare continually plagued the Axis occupiers.

Invasion of Soviet Union
Soviet Siberian Soldiers fighting during the Battle of Moscow.

Operation Barbarossa, the largest invasion in history, began June 22, 1941. An Axis force of over four million soldiers advanced rapidly deep into the Soviet Union, destroying almost the entire western Soviet army in huge battles of encirclement. The Soviets dismantled as much industry as possible ahead of the advancing forces, moving it to the Ural Mountains for reassembly. By late November, the Axis had reached a line at the gates of Leningrad, Moscow, and Rostov, at the cost of about 23 percent casualties. Their advance then ground to a halt. The German General Staff had underestimated the size of the Soviet army and its ability to draft new troops. They were now dismayed by the presence of new forces, including fresh Siberian troops under General Zhukov, and by the onset of a particularly cold winter. German forward units had advanced within distant sight of the golden onion domes of Moscow's Saint Basil's Cathedral, but then on December 5, the Soviets counter-attacked and pushed the Axis back some 150-250 kilometers (100-150 mi), which became the first major German defeat of World War II.

The Continuation War between Finland and the Soviet Union began on June 25, with Soviet air attacks shortly after the beginning of Operation Barbarossa.

Allied conferences

The Atlantic Charter was a joint declaration by Churchill and Roosevelt, August 14, 1941.

In late December 1941, Churchill met Roosevelt again at the Arcadia Conference. They agreed that defeating Germany had priority over defeating Japan. The Americans proposed a 1942 cross-channel invasion of France which the British strongly opposed, suggesting instead a small invasion in Norway or landings in French North Africa.

German paratroopers landing on the island of Crete.

Rommel's forces advanced rapidly eastward, laying siege to the vital seaport of Tobruk. Two Allied attempts to relieve Tobruk were defeated, but a larger offensive at the end of the year drove Rommel back after heavy fighting.

On May 20, the Battle of Crete began when elite German parachute and glider-borne mountain troops launched a massive airborne invasion of the Greek island. Crete was defended by Greek and Commonwealth troops. The Germans attacked the island's three airfields simultaneously. Their invasion on two airfields failed, but they successfully captured one, which allowed them to reinforce their position and capture the island in a little over one week.

In June 1941, Allied forces invaded Syria and Lebanon, capturing Damascus on June 17. In August, British and Soviet troops occupied neutral Iran to secure its oil and a southern supply line to Russia.

Pacific theater

Sino-Japanese war
Overview map of World War II in Asia and the Pacific: Allies green, Japanese conquests yellow.

A war had begun in East Asia before World War II started in Europe. On July 7, 1937, Japan, after occupying Manchuria in 1931, launched another attack against China near Beijing. The Japanese made initial advances but were stalled at Shanghai. The city eventually fell to the Japanese and in December 1937, the capital city Nanking (now Nanjing) fell. As a result, the Chinese government moved its seat to Chongqing for the rest of the war. The Japanese forces committed brutal atrocities against civilians and prisoners of war when Nanking was occupied, slaughtering as many as 300,000 civilians within a month. The war by 1940 had reached a stalemate with both sides making minimal gains. The Chinese had successfully defended their land from oncoming Japanese on several occasions while strong resistance in areas occupied by the Japanese made a victory seem impossible to the Japanese.

Japan and the United States
Pearl Harbor under attack on December 7, 1941

In the summer of 1941, the United States began an oil embargo against Japan, which was a protest to Japan's incursion into French Indo-China and the continued invasion of China. Japan planned an attack on Pearl Harbor to cripple the U.S. Pacific Fleet before consolidating oil fields in the Dutch East Indies. On December 7, a Japanese carrier fleet launched a surprise air attack on Pearl Harbor, Hawaii. The raid resulted in two U.S. battleships sunk, and six damaged but later repaired and returned to service. The raid failed to find any aircraft carriers and did not damage Pearl Harbor's usefulness as a naval base. The attack strongly united public opinion in the United States against Japan. The following day, December 8, the United States declared war on Japan. On the same day, China officially declared war against Japan. Germany declared war on the United States on December 11, even though it was not obliged to do so under the Tripartite Pact. Hitler hoped that Japan would support Germany by attacking the Soviet Union. Japan did not oblige, and this diplomatic move by Hitler proved a catastrophic blunder which unified the American public's support for the war.

Japanese offensive
Lt Gen Arthur Percival surrendering Singapore to the Japanese on February 15, 1942. It was the greatest defeat in British history.

Japan soon invaded the Philippines and the British colonies of Hong Kong, Peninsular Malaysia, Borneo, and Burma, with the intention of seizing the oilfields of the Dutch East Indies. Despite fierce resistance by American, Philippine, British, Canadian, and Indian forces, all these territories capitulated to the Japanese in a matter of months. The British island fortress of Singapore was captured in what Churchill considered one of the most humiliating British defeats of all time.

Deadlock: 1942

European theater

Western and Central Europe

In May, top Nazi leader Reinhard Heydrich was assassinated by Allied agents in Operation Anthropoid. Hitler ordered severe reprisals.

On August 19, British and Canadian forces launched the Dieppe Raid (codenamed Operation Jubilee) on the German occupied port of Dieppe, France. The attack was a disaster but provided critical information utilized later in Operation Torch and Operation Overlord.

Operation Blue: German advances from May 7 1942 to November 18 1942.
Soviet winter and early spring offensive

In the north, Soviets launched the Toropets-Kholm Operation January 9 to February 6 1942, trapping a German force near Andreapol. The Soviets also surrounded a German garrison in the Demyansk Pocket which held out with air supply for four months (February 8 until April 21), and established themselves in front of Kholm, Velizh and Velikie Luki.

In the south, Soviet forces launched an offensive in May against the German Sixth Army, initiating a bloody 17 day battle around Kharkov, which resulted in the loss of over 200,000 Red Army personnel.

Axis summer offensive

On June 28, the Axis began their summer offensive. German Army Group B planned to capture the city of Stalingrad, which would secure the German left while Army Group A planned to capture the southern oil fields. In the Battle of the Caucasus, fought in the late summer and fall of 1942, the Axis forces captured the oil fields.

Soviet soldiers fighting in the ruins of Stalingrad, 1942

After bitter street fighting which lasted for months, the Germans captured 90 percent of Stalingrad by November. The Soviets, however, had been building up massive forces on the flanks of Stalingrad. They launched Operation Uranus on November 19, with twin attacks that met at Kalach four days later and trapped the Sixth Army in Stalingrad. The Germans requested permission to attempt a break-out, which was refused by Hitler, who ordered Sixth Army to remain in Stalingrad where he promised they would be supplied by air until rescued. About the same time, the Soviets launched Operation Mars in a salient near the vicinity of Moscow. Its objective was to tie down Army Group Center and to prevent it from reinforcing Army Group South at Stalingrad.

In December, German relief forces got within 50 kilometers (30 mi) of the trapped Sixth Army before they were turned back by the Soviets. By the end of the year, the Sixth Army was in desperate condition, as the Luftwaffe was only able to supply about a sixth of the provisions needed. The battle ended in February 1943, when the Soviet forces succeeded in over-running the German positions.

The Battle of Stalingrad was a turning point in World War II and is considered the bloodiest battle in human history, with more combined casualties suffered than in any battle before. The battle was marked by brutality and disregard for military and civilian casualties on both sides. Total deaths are estimated to have approached 2.5 million. When it was over, the Axis powers had lost one fourth of their strength on that front.

Eastern North Africa
British infantry attack at the Second Battle of El Alamein.

At the beginning of 1942, the Allied forces in North Africa were weakened by detachments to the Far East. Rommel once again attacked and recaptured Benghazi. Then he defeated the Allies at the Battle of Gazala, and captured Tobruk with several thousand prisoners and large quantities of supplies. Following up, he drove deep into Egypt but with overstretched forces.

The First Battle of El Alamein took place in July 1942. Allied forces had retreated to the last defensible point before Alexandria and the Suez Canal. The Afrika Korps, however, had outrun its supplies, and the defenders stopped its thrusts. The Second Battle of El Alamein occurred between October 23 and November 3. Lieutenant-General Bernard Montgomery was in command of the Commonwealth forces, now known as the British Eighth Army. The Eighth Army took the offensive and was ultimately triumphant. After the German defeat at El Alamein, the Axis forces made a successful strategic withdrawal to Tunisia.

Western North Africa

Operation Torch, launched on November 8, 1942, aimed to gain control of Morocco and Algiers through simultaneous landings at Casablanca, Oran, and Algiers, followed a few days later with a landing at Bône, the gateway to Tunisia. It was hoped that the local forces of Vichy France (the puppet government in France under the Nazis) would put up no resistance and submit to the authority of Free French General Henri Giraud. In response, Hitler invaded and occupied Vichy France and Tunisia, but the German and Italian forces were caught in the pincers of a twin advance from Algeria and Libya. Rommel's victory against American forces at the Battle of the Kasserine Pass could only hold off the inevitable.

Pacific theater

Central and South West Pacific
American dive bombers over the burning Japanese cruiser Mikuma during the Battle of Midway.

On February 19, 1942, Roosevelt signed United States Executive Order 9066, leading to the internment of approximately 110,000 Japanese-Americans for the duration of the war.

In April, the Doolittle Raid, the first U.S. air raid on Tokyo, boosted morale in the U.S. and caused Japan to shift resources to homeland defense, but did little actual damage.

In early May, a Japanese naval invasion of Port Moresby, New Guinea, was thwarted by Allied navies in the Battle of the Coral Sea. This was both the first successful opposition to a Japanese attack and the first battle fought between aircraft carriers.

On June 5, American carrier-based dive-bombers sank four of Japan's best aircraft carriers in the Battle of Midway. Historians mark this battle as a turning point and the end of Japanese expansion in the Pacific. Cryptography played an important part in the battle, as the United States had broken the Japanese naval codes and knew the Japanese plan of attack.

In July, a Japanese overland attack on Port Moresby was led along the rugged Kokoda Track. An outnumbered and untrained Australian battalion defeated the 5,000-strong Japanese force, the first land defeat of Japan in the war and one of the most significant victories in Australian military history.

U.S. Marines rest in the field on Guadalcanal, circa August-December 1942

On August 7, United States Marines began the Battle of Guadalcanal. For the next six months, U.S. forces fought Japanese forces for control of the island. Meanwhile, several naval encounters raged in the nearby waters, including the Battle of Savo Island, Battle of Cape Esperance, Naval Battle of Guadalcanal, and Battle of Tassafaronga. In late August and early September, while battle raged on Guadalcanal, an amphibious Japanese attack on the eastern tip of New Guinea was met by Australian forces in the Battle of Milne Bay.

Sino-Japanese War

Japan launched a major offensive in China following the attack on Pearl Harbor. The aim of the offensive was to take the strategically important city of Changsha which the Japanese had failed to capture on two previous occasions. For the attack, the Japanese massed 120,000 soldiers under 4 divisions. The Chinese responded with 300,000 men, and soon the Japanese army was encircled and had to retreat.

War turns: 1943

European theater

German and Soviet spring offensives
German advances during the spring and summer of 1943.

After the surrender of the German Sixth Army at Stalingrad on February 2, 1943, the Red Army launched eight offensives during the winter. Many were concentrated along the Don basin near Stalingrad, which resulted in initial gains until German forces were able to take advantage of the weakened condition of the Red Army and regain the territory it lost.

Operation Citadel

On July 4, the Wehrmacht launched a much-delayed offensive against the Soviet Union at the Kursk salient. Their intentions were known by the Soviets, and they hastened to defend the salient with an enormous system of earthwork defenses. Both sides massed their armor for what became a decisive military engagement. The Germans attacked from both the north and south of the salient and hoped to meet in the middle, cutting off the salient and trapping 60 Soviet divisions. The German offensive was ground down as little progress was made through the Soviet defenses. The Soviets then brought up their reserves, and the largest tank battle of the war occurred near the city of Prokhorovka. The Germans had exhausted their armored forces and could not stop the Soviet counter-offensive that threw them back across their starting positions.

Soviet fall and winter offensives

In August, Hitler agreed to a general withdrawal to the Dnieper line, and as September proceeded into October, the Germans found the Dnieper line impossible to hold as the Soviet bridgeheads grew. Important Dnieper towns started to fall, with Zaporozhye the first to go, followed by Dnepropetrovsk.

Early in November the Soviets broke out of their bridgeheads on either side of Kiev and recaptured the Ukrainian capital.

The First Ukrainian Front attacked at Korosten on Christmas Eve. The Soviet advance continued along the railway line until the 1939 Polish-Soviet border was reached.

The U.S. support ship Robert Rowan explodes after being hit by a German bomber near Gela, Sicily on 11 July 1943

The surrender of Axis forces in Tunisia on May 13, 1943 yielded some 250,000 prisoners. The North African war proved to be a disaster for Italy, and when the Allies invaded Sicily on July 10, in Operation Husky, capturing the island in a little over a month, the regime of Benito Mussolini collapsed. On July 25, he was removed from office by the King of Italy, and arrested with the positive consent of the Great Fascist Council. A new government, led by Pietro Badoglio, took power but declared that Italy would stay in the war. Badoglio actually had begun secret peace negotiations with the Allies.

The Allies invaded mainland Italy on September 3, 1943. Italy surrendered to the Allies on September 8, as had been agreed in negotiations. The royal family and Badoglio government escaped to the south, leaving the Italian army without orders, while the Germans took over the fight, forcing the Allies to a complete halt in the winter of 1943-44 at the Gustav Line south of Rome.

In the north, the Nazis let Mussolini create what was effectively a puppet state, the Italian Social Republic or "Republic of Salò," named after the new capital of Salò on Lake Garda.

Mid-1943 brought the fifth and final German Sutjeska offensive against the Yugoslav Partisans.

Pacific theater

Central and South West Pacific
Battleship USS Pennsylvania (BB38) leading USS Colorado (BB-45), cruisers USS Louisville (CA-28), USS Portland (CA-33) and light cruiser USS Columbia (CL-56) into Lingayen Gulf, Philippines, January 1945.<>