Britain had been growing increasingly concerned about Japanese expansionism, and had made several attempts to negotiate a peaceful settlement. However, these attempts had failed, and by the end of 1941 it was clear that war was inevitable.
Britain's declaration of war against Japan occurred as part of a wider declaration of war by the Allies, which included the United States, China, and the Netherlands. The declaration was made on December 8, 1941, one day after the attack on Pearl Harbour.
The immediate consequence of Britain's declaration of war against Japan was the outbreak of hostilities in the Far East. British forces were involved in fighting against Japanese forces in Burma, Malaya, and Singapore.