Primary Source Sets
World War II: America's Motivation and Impact

World War II: America's Motivation and Impact

How did the United States’ motivations for entering World War II impact our actions?

Following World War I, the United States hoped to avoid further entanglement with European politics that had drawn us into war. A strong isolationist sentiment developed that questioned the wisdom of our entry into The Great War as it was then known. However, the rise of military government in Germany, Italy and Japan and their invasions of neighboring countries became a major concern for United States leaders including President Franklin Delano Roosevelt.

Germany Instigates World War I

In Europe, Adolf Hitler led the rise of the Nazi Party, which claimed that Germany was treated unfairly in the peace treaty that ended WWI. He also sought to unite all German-speaking peoples, a policy that put him at odds with several neighbors like Austria, Poland and Czechoslovakia. Great Britain and France tried to negotiate an end to German expansion, but the Soviet Union on Germany’s eastern front signed a non-aggression treaty with Hitler that opened the door to Germany’s invasion of Poland in 1939. France and England came to the aid of the Poles and declared war on Germany. Hitler’s armies quickly overran Poland and then France, leaving Britain alone against German armies and air force. President Roosevelt wanted to come to the aid of our British allies, but public sentiment was not yet ready to send American soldiers to fight in another European war.

Meanwhile, Germany and Italy became partners with Japan that had designs on domination of Eastern Asia. Japan lacked natural resources like oil and rubber and created plans to attack neighboring countries that could supply them. They invaded Korea and Manchuria and then China. They also looked southward to the European colonies of Dutch East Asia and British Malaysia. They knew that the United States and Great Britain would fight to stop them. To weaken U.S. naval forces in the Pacific, Japan bombed the naval base at Pearl Harbor in Hawaii on December 7, 1941. America declared war on Japan, and on December 11, Germany and Italy lived up to their agreement with Japan and declared war on the United States. Iowan Henry A. Wallace had been elected vice president in 1940 and served there throughout most of the war.

American Offensive in European and Pacific Fronts

Instead of putting all its efforts to fight Japan, the United States made Europe its first priority. Roosevelt met with Winston Churchill, the British prime minister, and they agreed that Hitler was a greater danger than Japan. German planes were bombing London regularly, and many expected a Nazi invasion. The United States began mobilizing armies, converting its factories to produce war supplies, and encouraging farmers to boost production. British and American generals developed a plan to invade Europe through Italy before attempting an attack across the English Channel against heavily fortified defenses. Meanwhile, German armies had invaded the Soviet Union and were imposing frightening losses on military and civilian populations alike. The Soviets, with the aid of a brutal Russian winter, halted the Nazi advance and forced a German retreat. Finally, in June 1944, a combined American-British invasion force landed on the French coast of Normandy, established a beach head, and from there began an offensive that led to a German surrender in May 1945.

Meanwhile, the U.S. Navy took the offensive in the Pacific against Japan. The route to Japan led through several Pacific Islands that the Japanese defended with determination. Two Allied naval victories broke the strength of the Japanese fleet and allowed the Allied forces to get close enough to establish air bases from which bombers could strike Japanese cities. The estimates of the loss of life that would be required to force the main Japanese islands to surrender reached a million. During the war, in a very secret project, U.S. scientists had developed a bomb that was hundreds of times more powerful than anything before. In August, 1945, President Harry Truman ordered atomic bombs to be dropped on the cities of Hiroshima and Nagasaki, bringing the world into the nuclear age. Japan surrendered within a matter of days, and WWII was over.

Some estimates of the loss of life due to fighting, disease and other war-related factors run as high as 60 million, or about 3 percent of the world's population at the time. The Soviet Union suffered the greatest cost, with some 20 million civilian and military casualties. The United States, protected by two oceans from the battlefields, sustained around 420,000 war-related deaths. Iowa soldiers killed or wounded are recorded as around 2,800.

Supporting Questions

Why did the US get involved in World War II?

Why did America fight differently in the two theaters?

How was victory achieved on each front?