US–Japan relations

Hitler's invasion of Russia was perhaps the decisive move in World War II. Abandoning any attempt to invade the United Kingdom, he switched his attention to the East and, in June 1941, threw a vast army into the Soviet Union. He failed to gain the decisive victory that he sought; this time blitzkrieg did not work. But that was only the more obvious effect. The move also transformed the position in East Asia. Rather than joining in against the Russians, Japan used their involvement in war as an opportunity to turn south. That move provoked sanctions from the US as well as from the UK and the Netherlands. Sanctions did not bring Japan to a halt. Instead the Japanese resolved on attacking both Britain and the US. That decision precipitated US entry into the war, particularly as Hitler declared war on the US as well.

Before the invasion of Russia the Japanese had remained cautious. There were alarms and excursions, but although the Japanese gained some advantages by their mediation in the Thailand–Indo-China conflict, they appeared to bide their time, awaiting, it was thought, some decisive German move against the British. Their caution perhaps added to the image the West held of their relative weakness and intensified the belief that they would not attempt to attack if the US and the UK showed sufficient resolve. That belief proved to be mistaken.

The British had little room to manoeuvre.