13.1 An Unstable Peace

Causes of World War II

World War II grew out of a volatile mix of nationalism, militarism, and territorial ambitions that festered during the 1920s and 1930s. Germany, Italy, and Japan each pursued aggressive expansion driven by extreme ideologies, while Britain and France tried to keep the peace through concessions. When those concessions failed, the result was the deadliest conflict in human history.

The United States initially stayed out, committed to isolationism after the costly experience of World War I. But as the Axis powers expanded, America gradually shifted toward supporting the Allies. Japan's surprise attack on Pearl Harbor in December 1941 finally brought the U.S. into full-scale war.

Nationalism, Militarism, and Territorial Expansion

Extreme nationalism surged in Germany, Italy, and Japan after World War I, but each country's version looked different.

• Germany felt humiliated by the Treaty of Versailles (1919), which stripped it of territory, imposed massive reparations, and severely limited its military. Hitler and the Nazi Party channeled that resentment into a vision of restoring German greatness and uniting all German-speaking peoples under one state.
• Italy under Mussolini embraced fascism, which glorified the state above the individual and called for territorial expansion to recreate the glory of ancient Rome.
• Japan sought to dominate East Asia, secure raw resources it lacked domestically (especially oil and rubber), and establish itself as a major colonial power on par with European empires.

Militarism reinforced these nationalist movements. Hitler rearmed Germany in direct violation of the Versailles Treaty's restrictions on military size and weapons. In Japan, the military gained increasing control over the civilian government during the 1930s and promoted conquest as the solution to economic problems like resource scarcity.

These ideologies translated into concrete territorial aggression:

• Hitler annexed Austria in 1938 (the Anschluss) and demanded Czechoslovakia's Sudetenland, aiming to create a "Greater Germany" and eventually gain Lebensraum (living space) in Eastern Europe.
• Mussolini invaded Ethiopia in 1935 to build a "New Roman Empire" in Africa, partly to avenge Italy's embarrassing defeat there in 1896.
• Japan invaded Manchuria in 1931 and launched a full-scale invasion of China in 1937, seeking to dominate the region and control its resources.

Appeasement

Britain and France responded to German aggression with a policy of appeasement, making concessions to Hitler in hopes of avoiding another devastating war.

Key moments of appeasement:

1. Rhineland (1936): Hitler remilitarized the Rhineland, a border region that the Versailles Treaty had required to remain demilitarized. Britain and France protested but took no action.
2. Austria (1938): Hitler annexed Austria. Again, the Western powers did not intervene.
3. Munich Conference (1938): British Prime Minister Neville Chamberlain and French leaders met with Hitler and agreed to let Germany annex the Sudetenland in exchange for a promise of no further expansion. Chamberlain famously returned home declaring "peace for our time."

Why did Britain and France appease Hitler? Several factors were at play:

• Many leaders believed some of Germany's grievances about the Versailles Treaty were legitimate and that addressing them could stabilize Europe.
• Both countries were still recovering from WWI's devastation and feared that confronting Hitler would trigger another war they were unprepared to fight.
• There was genuine hope that limited concessions would satisfy Hitler's demands.

Appeasement ultimately failed and arguably made war more likely. It emboldened Hitler, convincing him that the Western powers were weak and unwilling to fight. It gave Germany critical time to rebuild its military. And it led Hitler to believe he could invade Poland in September 1939 without facing serious opposition. He was wrong on that last point: Britain and France declared war on Germany two days after the invasion, and World War II began.

U.S. Entry into World War II

From Isolationism to Intervention

After World War I, the United States retreated into isolationism, determined to avoid entanglement in another European conflict. The Neutrality Acts of the mid-1930s banned arms sales and loans to nations at war. Many Americans felt that involvement in WWI had been a mistake driven by propaganda and profit, and with the Great Depression still causing hardship at home, foreign wars seemed like someone else's problem.

President Franklin Roosevelt, however, recognized the growing threat of the Axis powers and began nudging the country away from strict neutrality:

1. "Cash and Carry" (1939): Allied nations could purchase American weapons, but only if they paid upfront and transported the goods themselves. This technically maintained neutrality while clearly favoring Britain and France, since Germany's navy couldn't safely cross the Atlantic.
2. Lend-Lease Act (1941): The U.S. could lend or lease military supplies to any nation whose defense was considered vital to American security. This was a major step away from neutrality and kept Britain and the Soviet Union fighting while the U.S. stayed officially out of combat.

Meanwhile, tensions with Japan escalated. The U.S. opposed Japan's invasion of China (1937) and its move into French Indochina (1940), viewing both as threats to American interests and the Open Door Policy in Asia. In 1941, the U.S. imposed an oil embargo on Japan, cutting off roughly 80% of Japan's oil supply. Japanese leaders saw this as an existential threat and began planning a preemptive strike.

On December 7, 1941, Japan launched a surprise attack on the U.S. naval base at Pearl Harbor, Hawaii, killing over 2,400 Americans and destroying much of the Pacific fleet. President Roosevelt called it "a date which will live in infamy," and Congress declared war on Japan the next day. Germany and Italy then declared war on the United States as part of their Axis alliance.

Pearl Harbor transformed American public opinion overnight. The U.S. mobilized its massive industrial and military capacity for total war, fundamentally shifting the balance of power against the Axis.