World War II (WWII or WW2), also known as the Second World War, was a global war. It is generally considered to have lasted from 1939 to 1945, although some conflicts in Asia that are commonly viewed as becoming part of the world war had begun earlier than 1939.
It involved the vast majority of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis. It was the most widespread war in history, and directly involved more than 100 million people, from more than 30 different countries. In a state of "total war", the major participants threw their entire economic, industrial, and scientific capabilities behind the war effort, erasing the distinction between civilian and military resources. Marked by mass deaths of civilians, including the Holocaust, the Three Alls Policy, the strategic bombing of enemy industrial and/or population centers, and the first use of nuclear weapons in combat, it resulted in an estimated 50 million to 85 million fatalities. These made World War II the deadliest conflict in human history.
The Empire of Japan aimed to dominate East Asia and was already at war with the Republic of China in 1937, but the world war is generally said to have begun on 1 September 1939 with the invasion of Poland by Germany and subsequent declarations of war on Germany by France and the United Kingdom. From late 1939 to early 1941, in a series of campaigns and treaties, Germany formed the Axis alliance with Italy, conquering or subduing much of continental Europe. Following the Molotov–Ribbentrop Pact, Germany and the Soviet Union partitioned and annexed territories between themselves their European neighbours, including Poland, Finland and the Baltic states. The United Kingdom and the other members of the British Commonwealth were the only major Allied forces continuing the fight against the Axis, with battles taking place in North Africa and the Horn of Africa as well as the long-running Battle of the Atlantic. In June 1941, the European Axis powers launched an invasion of the Soviet Union, giving a start to the largest land theatre of war in history, which tied down the major part of the Axis' military forces for the rest of the war. In December 1941, Japan joined the Axis, attacked the United States and European territories in the Pacific Ocean, and quickly conquered much of the Western Pacific.
The Axis advance was stopped in 1942, when Japan lost a critical battle at Midway, near Hawaii, and Germany was defeated in North Africa and then decisively, at Stalingrad in Russia. In 1943, with a series of German defeats on the Eastern Front, the Allied invasion of Italy which brought about that nation's surrender, and American victories in the Pacific, the Axis lost the initiative and undertook strategic retreat on all fronts. In 1944, the Western Allies invaded France, while the Soviet Union regained all of its territorial losses and invaded Germany and its allies. During 1944 and 1945 the Japanese began suffering major reverses in mainland Asia in Burma and South Central China whilst the United States defeated the Japanese Navy and captured key Western Pacific islands.
The war in Europe ended with an invasion of Germany by the Western Allies and the Soviet Union culminating in the capture of Berlin by Soviet and Polish troops and the subsequent German unconditional surrender on 8 May 1945. Following the Potsdam Declaration by the Allies on 26 July 1945, the United States dropped atomic bombs on the Japanese cities of Hiroshima and Nagasaki on 6 August and 9 August respectively. With an invasion of the Japanese archipelago (known as Operation Downfall) imminent, and the Soviet Union's declaration of war on Japan with the invasion of Manchuria, Japan surrendered, 15 August 1945. Thus ended the war in Asia, cementing the total victory of the Allies over the Axis.
World War II altered the political alignment and social structure of the world. The United Nations (UN) was established to foster international co-operation and prevent future conflicts. The great powers that were the victors of the war—the United States, the Soviet Union, China, the United Kingdom, and France—became the permanent members of the United Nations Security Council. The Soviet Union and the United States emerged as rival superpowers, setting the stage for the Cold War, which lasted for the next 46 years. Meanwhile, the influence of European great powers started to decline, while the decolonisation of Asia and Africa began. Most countries whose industries had been damaged moved towards economic recovery. Political integration, especially in Europe, emerged as an effort to stabilise postwar relations and cooperate more effectively in the Cold War.
The start of the war is generally held to be 1 September 1939, beginning with the German invasion of Poland; Britain and France declared war on Germany two days later. Other dates for the beginning of war include the start of the Second Sino-Japanese War on 7 July 1937.
Others follow the British historian A. J. P. Taylor, who held that the Sino-Japanese War and war in Europe and its colonies occurred simultaneously and the two wars merged in 1941. This article uses the conventional dating. Other starting dates sometimes used for World War II include the Italian invasion of Abyssinia on 3 October 1935. The British historian Antony Beevor views the beginning of the Second World War as the Battles of Khalkhin Gol fought between Japan and the Mongolia, Soviet Union from May to September 1939.
The exact date of the war's end is also not universally agreed upon. It has been suggested that the war ended at the armistice of 14 August 1945 (V-J Day), rather than the formal surrender of Japan (2 September 1945); in some European histories, it ended on V-E Day (8 May 1945). However, the Treaty of Peace with Japan was not signed until 1951, and that with Germany not until 1990.
World War I had radically altered the political map, with the defeat of the Central Powers—including Austria-Hungary, Germany and the Ottoman Empire—and the 1917 Bolshevik seizure of power in Russia. Meanwhile, existing victorious Allies such as France, Belgium, Italy, Greece and Romania gained territories, whereas new states were created out of the collapse of Austria-Hungary and the Russian and Ottoman Empires.
Despite the pacifist movement in the aftermath of the war, the losses still caused irredentist and revanchist nationalism to become important in a number of European states. Irredentism and revanchism were strong in Germany because of the significant territorial, colonial, and financial losses incurred by the Treaty of Versailles. Under the treaty, Germany lost around 13 percent of its home territory and all of its overseas colonies, while German annexation of other states was prohibited, reparations were imposed, and limits were placed on the size and capability of the country's armed forces. Meanwhile, the Russian Civil War had led to the creation of the Soviet Union.
The German Empire was dissolved in the German Revolution of 1918–1919, and a democratic government, later known as the Weimar Republic, was created. The interwar period saw strife between supporters of the new republic and hardline opponents on both the right and left. Although Italy as an Entente ally made some territorial gains, Italian nationalists were angered that the promises made by Britain and France to secure Italian entrance into the war were not fulfilled with the peace settlement. From 1922 to 1925, the Fascist movement led by Benito Mussolini seized power in Italy with a nationalist, totalitarian, and class collaborationist agenda that abolished representative democracy, repressed socialist, left-wing and liberal forces, and pursued an aggressive foreign policy aimed at forcefully forging Italy as a world power, promising the creation of a "New Roman Empire".
In Germany, the Weimar Republic's legitimacy was challenged by right-wing elements such the Freikorps and the Nazi party, resulting in events such as the Kapp Putsch and the Beer Hall Putsch. With the onset of the Great Depression in 1929, domestic support for Nazism and its leader Adolf Hitler rose and, in 1933, he was appointed Chancellor of Germany. In the aftermath of the Reichstag fire, Hitler created a totalitarian single-party state led by the Nazis.
The Kuomintang (KMT) party in China launched a unification campaign against regional warlords and nominally unified China in the mid-1920s, but was soon embroiled in a civil war against its former Chinese communist allies. In 1931, an increasingly militaristic Japanese Empire, which had long sought influence in China as the first step of what its government saw as the country's right to rule Asia, used the Mukden Incident as a pretext to launch an invasion of Manchuria and establish the puppet state of Manchukuo.
Too weak to resist Japan, China appealed to the League of Nations for help. Japan withdrew from the League of Nations after being condemned for its incursion into Manchuria. The two nations then fought several battles, in Shanghai, Rehe and Hebei, until the Tanggu Truce was signed in 1933. Thereafter, Chinese volunteer forces continued the resistance to Japanese aggression in Manchuria, and Chahar and Suiyuan.
Adolf Hitler, after an unsuccessful attempt to overthrow the German government in 1923, eventually became the Chancellor of Germany in 1933. He abolished democracy, espousing a radical, racially motivated revision of the world order, and soon began a massive rearmament campaign. It was at this time that multiple political scientists began to predict that a second Great War might take place. Meanwhile, France, to secure its alliance, allowed Italy a free hand in Ethiopia, which Italy desired as a colonial possession. The situation was aggravated in early 1935 when the Territory of the Saar Basin was legally reunited with Germany and Hitler repudiated the Treaty of Versailles, accelerated his rearmament programme and introduced conscription.
Hoping to contain Germany, the United Kingdom, France and Italy formed the Stresa Front; however, in June 1935, the United Kingdom made an independent naval agreement with Germany, easing prior restrictions. The Soviet Union, concerned due to Germany's goals of capturing vast areas of eastern Europe, wrote a treaty of mutual assistance with France. Before taking effect though, the Franco-Soviet pact was required to go through the bureaucracy of the League of Nations, which rendered it essentially toothless. The United States, concerned with events in Europe and Asia, passed the Neutrality Act in August. In October, Italy invaded Ethiopia, and Germany was the only major European nation to support the invasion. Italy subsequently dropped its objections to Germany's goal of absorbing Austria.
Hitler defied the Versailles and Locarno treaties by remilitarising the Rhineland in March 1936. He received little response from other European powers. When the Spanish Civil War broke out in July, Hitler and Mussolini supported the fascist and authoritarian Nationalist forces in their civil war against the Soviet-supported Spanish Republic. Both sides used the conflict to test new weapons and methods of warfare, with the Nationalists winning the war in early 1939. In October 1936, Germany and Italy formed the Rome–Berlin Axis. A month later, Germany and Japan signed the Anti-Comintern Pact, which Italy would join in the following year. In China, after the Xi'an Incident the Kuomintang and communist forces agreed on a ceasefire in order to present a united front to oppose Japan.