Franklin D. Roosevelt – United States President at the time - believed that war with the aggressor countries was unavoidable and wanted to assist Great Britain, Canada and their allies. But millions of Americans were against any involvement and insisted isolationism. If he wanted to be re-elected, the President had to be very careful about not involving the US in conflicts that did not directly involve its interests.

Japan was hungry for more land and natural resources to supply its war factories; it wanted control over all of the countries and lands in the Pacific region. But the United States stood in the way of these ambitions. In 1940 the United States stopped all exports to Japan of iron, steel, and other military supplies that might be used to realize Japanese imperialist dreams. However, the United States was 9600km from Asia. The Japanese military leaders began to plan ways to destroy the American navy in the Pacific Ocean. This would allow Japan to attack and conquer the other countries around them - such as Korea and China - without too much effort. Emperor Hirohito of Japan opposed war with the United States, but military leaders had control of the government and the Emperor had next-to-nothing influence on the choices made by Japan.

On December 7th, 1941, Japan lauched an air raid from Japanese aircraft carriers against the US Naval headquarters at Pearl Harbor, Hawaii. The Americans were not expecting the attack, which involved 360 Japanese fighter planes and followed the tactics of Hitler’s blitzkrieg. Most of the US fighter planes were destroyed as they sat defenceless on their runways and three American destroyers were sunk in the harbour. Altogether 2500 Americans were killed and 1200 were injured. The American navy was crippled temporaily.

Fortunately for the United States, three of their aircraft carriers were at sea when the attack took place;  this would be essential for the counter-attack against Japan. The attack on Pearl Harbor would actually prove to be a deadly mistake for the Japanese later on.

The day after the attack on Pearl Harbor, the United States declared war on Japan. It had taken a direct enemy attack on American soil to silence the isolationists. Three days later as part of the Axis Alliance Agreement among Germany, Italy, and Japan, Germany and Italy declared war on the United States; the most powerful country in the world was now fighting on the side of the Allies.