Why Did World War II Start?

How did the Treaty of Versailles and the Depression set the stage?

The Treaty of Versailles was signed on June 28, 1919 ^{[8] [5]}. It held Germany responsible for starting World War I and liable for massive damages. Germany lost about 13 percent of its territory and about 10 percent of its population, its army was capped at 100,000 men, conscription was banned, and the Rhineland was demilitarized ^[3]. The Office of the Historian, counting only prewar territory in Europe, gives a figure of about 10 percent. In 1921 an Allied commission set reparations at 132 billion gold Reichsmarks, about 32 billion U.S. dollars ^[9]. The U.S. Senate never ratified the treaty: a final vote on March 19, 1920 fell seven votes short.

Many Germans called it a "dictated peace" ^[5]. Revising the treaty became one of the platforms that gave radical right-wing parties, including the Nazis, credibility with mainstream voters ^[3].

Economic crisis did the rest. Hyperinflation hit Germany by 1923, and the Great Depression, which began in 1929, wiped out middle-class savings and caused mass unemployment ^[3]. You can read more about that collapse in why the Great Depression happened. The Nazi vote climbed as the crisis deepened:

Election	Nazi share of the vote
Reichstag, May 20, 1928	2.6%
Reichstag, September 14, 1930	18%
Reichstag, July 31, 1932	37% (largest party)
Reichstag, November 6, 1932	33%
Reichstag, March 5, 1933 (after Hitler took office)	almost 44%

Figures from the U.S. Holocaust Memorial Museum ^[4]. The Nazis never won a majority of the vote in these elections. Adolf Hitler was appointed chancellor on January 30, 1933. The Enabling Act of March 23, 1933 let the government make laws without parliament, and by July 1933 the Nazi Party was the only legal party ^[4].

Versailles and the Depression did not make war certain, but they gave Hitler the anger and the audience he needed to take power.

What did Hitler want, and why did it mean war?

Hitler's foreign policy had two main goals: to unite all Germans in a Nazi German empire and to acquire Lebensraum, "living space," in eastern Europe ^[6]. Those goals were bound up with the regime's racial and antisemitic ideology. The U.S. Holocaust Memorial Museum writes that the Nazis began planning a European war from the day they came to power in late January 1933 ^[2].

Rearmament came first. On March 16, 1935, a new law brought back the draft and expanded the German army to 550,000 men, openly breaking the Versailles limits ^[5].

War and genocide were tied together. As German troops occupied more territory, the regime's racial and antisemitic policies moved from persecution to genocide. Jews were the primary victims of the Holocaust: six million were murdered ^[2].

Hitler's war was not only a contest for borders; it was built on a racial ideology that led to the Holocaust.

How did Japan and Italy start wars of their own?

War came to Asia years before Poland. On September 18, 1931, an explosion damaged a section of Japanese-owned railway near Mukden. Japan blamed Chinese nationalists and used the blast to invade Manchuria, then declared the region a new state called Manchukuo, which the local Japanese army controlled ^[10]. When the League of Nations adopted a report that refused to recognize Manchukuo in 1933, the Japanese delegation walked out and never returned to the League Council.

On July 7, 1937, Chinese and Japanese forces clashed at the Marco Polo Bridge near Beijing, which threw the two nations into a full-scale war ^[11]. The U.S. Holocaust Memorial Museum treats Japan's invasion of China proper in July 1937 as the start of World War II in Asia, and its key-dates timeline of the war opens with the 1931 invasion of Manchuria ^{[2] [1]}. So "1939" is a European answer; for China the war had already been going on for two years.

Italy under Benito Mussolini also chose conquest. Fascist Italy invaded Ethiopia on October 2, 1935, and annexed it by May 1936. Germany and Italy announced the Rome-Berlin Axis on November 1, 1936, and Germany and Japan signed the Anti-Comintern Pact on November 25, 1936; Italy joined it in November 1937. Italy then seized Albania in April 1939 ^[1].

Why did Britain and France appease Hitler?

Between 1936 and 1939 Germany took one step after another without meeting armed resistance. In March 1936 the new Wehrmacht remilitarized the Rhineland ^[5]. From March 11 to 13, 1938, Germany absorbed Austria, the Nazi regime's first act of territorial expansion. The U.S. Holocaust Memorial Museum calls the other powers' acceptance of it "a significant act of appeasement" ^[6].

Next came Czechoslovakia. About six months after the Anschluss, Nazi Germany "manufactured a crisis" in the Sudetenland ^[6], a region of Czechoslovakia with about three million Germans. British Prime Minister Neville Chamberlain met Hitler at Berchtesgaden, then at Godesberg, and finally at Munich on September 29, 1938 ^[13]. Chamberlain and French Premier Édouard Daladier signed the Munich Agreement early on September 30. That afternoon, outside 10 Downing Street, Chamberlain said: "I believe it is peace for our time" ^[12].

Why give way? The National WWII Museum lists a strong wish to keep the peace after the devastation of World War I, the fact that Britain and France were not militarily ready to fight for Czechoslovakia in 1938, and fear that German bombers could devastate British cities ^[12].

Munich did not hold. In March 1939 Germany broke the agreement and occupied the Czech lands, including Prague ^[6]. Britain and France then guaranteed Poland's borders on March 31, 1939 ^[1].

What do historians argue about?

Two debates shape how historians explain the war's origins.

Was appeasement a blunder or a reasonable choice? The National Archives of the United Kingdom calls the Munich Agreement "a major failure of the policy of appeasement" that encouraged further aggression ^[13]. Tim Bouverie's Appeasing Hitler (2019) is critical too: it calls the failure to grasp the nature of the Nazi regime "the single greatest failure of British policy makers during this period." Defenders of the appeasers point to Anglo-French unpreparedness, fear of a divided public and empire, and the argument that Hitler's bad faith only became clear in March 1939. Bouverie rejects the military argument, holding that the Allies had strategic advantages in 1938, while reviewer Adam Timmins finds the public-opinion and hindsight arguments more persuasive ^[15].

How much did Hitler plan? "Intentionalist" historians see Nazi policy as flowing from Hitler's long-held aims. "Structuralist" (or "functionalist") historians focus on how the regime's competing agencies and leaders pushed policy forward, and Hans Mommsen famously called Hitler in some respects a "weak dictator." Ian Kershaw, whose Hitler biography leans toward the structuralist side, argued that officials were "working towards the Führer," acting on his broad goals without detailed orders ^[16]. The debate was sharpest over the Holocaust, and by the early 2000s many scholars considered the old dichotomy "either exhausted or simply passé" ^[17]. Most historians now combine both views: Hitler's ideology set the direction, and the regime's structure drove the pace.

Historians still disagree about how much blame belongs to the appeasers, but not about who started the war in Europe: Nazi Germany chose to invade Poland.

How did the invasion of Poland start the war in Europe?

On August 23, 1939, Nazi Germany and the Soviet Union signed a non-aggression pact. Its secret protocol divided eastern Europe into German and Soviet spheres and agreed to split Poland along the Narev, Vistula and San rivers ^[7]. Without fear of Soviet intervention, Germany invaded Poland on September 1, 1939. Britain and France declared war on Germany on September 3. The Soviet Union invaded Poland from the east on September 17, and on September 29 the two powers divided the country.

The table below shows where the main powers stood in September 1939. Italy did not enter the war until June 10, 1940 ^[1].

The war widened quickly. By June 1940 Germany had attacked France and the Low Countries, and France signed an armistice on June 22. On September 27, 1940, Germany, Italy and Japan signed the Tripartite Pact ^[1], which linked the wars in Europe and Asia ^[11]. On June 22, 1941, Germany and its partners invaded the Soviet Union.

Why did the United States enter the war in 1941?

The United States had been the main supplier of the oil, steel and iron that Japan's military needed. As Japan's war in China went on, the U.S. moved toward an embargo, and in mid-1941 it imposed a full embargo on exports to Japan and froze Japanese assets in American banks ^[11].

Japanese planes bombed the U.S. fleet at Pearl Harbor on December 7, 1941 ^[11]. At 12:30 p.m. on December 8, President Franklin D. Roosevelt addressed a joint session of Congress. The Senate voted unanimously for war; in the House, only Representative Jeannette Rankin of Montana voted no. Roosevelt signed the declaration at 4:00 p.m. that afternoon ^[14]. On December 11, Germany and Italy declared war on the United States ^[2]. Roosevelt's place in the line of presidents is in our list of U.S. presidents in order.

Pearl Harbor turned two regional wars, one in Europe and one in Asia, into a single world war.

For the war that came before, see why World War I started; for what came after, see why the Cold War started. More stories from this period are in the World Wars era and our why history happened collection.