On March 16, 1935, Adolf Hitler began to actively promote Germany's role as an aggressor state. Ignoring the guidelines of the Treaty of Versailles, he ordered the rearmament of Germany. Though Britain and France issued formal complaints, no action was taken to prevent Germany from rebuilding its arsenals and issuing conscription orders. In 1936, Germany began seeking allies. The Anti-Comintern Pact was signed between Germany and the Empire of Japan in November 1936. Less than a year later, Italy joined the Pact under Benito Mussolini's rule, thereby cementing the Axis powers. On March 12, 1938, Austria was annexed by Germany's Third Reich in an event now known as the Anschluss. Though no violence occurred during the event, it marked an important moment for German expansionism and the advancement of Axis goals.

By September 1939, Germany had annexed Sudetenland, a German-speaking region of Czechoslovakia, and invaded Poland. Hitler was expanding his giant war machine, regaining territories lost in World War I, and consolidating the ethnic German population. He was preparing to seize Europe, and the continent could only watch in horror. After the fall of Poland, a half-dozen countries declared neutrality and the British Armed Forces began preparing to evacuate civilians from major cities in the United Kingdom.

In the United States, the response was more muted. There were rumblings of war, but the Atlantic proved to be a barrier to the panic washing through Europe. On September 5, the U.S. proclaimed its neutrality and looked on as the first Jewish ghetto was established in Poland. As the situation worsened, however, the U.S. could no longer ignore the threat posed by Hitler. Despite an outcry from American isolationists, the U.S. Neutrality Act was passed on November 4, allowing the French and British to buy arms from the United States on a cash-only basis.

The Nazi army, by spring 1940, had begun to reach into Northern Europe, eventually seizing Norway. Bombings commenced throughout the United Kingdom and Paris, the Russian military became mired in conflict in the Baltic states, and the Japanese engaged in attacks on China. Hostilities worsened on June 10 when Italy declared war on France and Great Britain, opening yet another front in the war. While the United States maintained its official stance of neutrality, the country knew war was only a short time away.