British declaration of war on Japan
The government of the United Kingdom declared war on the Empire of Japan on 8 December 1941, following the Japanese attacks on British Malaya, Singapore and Hong Kong on the previous day, as well as in response to the bombing of the American fleet at Pearl Harbor.
Background
The United Kingdom [British United States declaration of war on Japan|declaration of war on Germany (1939)|declared war on Nazi Germany] on 3 September 1939, two days after the outbreak of war in Europe. The Empire of Japan and Nazi Germany had signed the Anti-Comintern Pact in 1936, to counter the perceived threat of the communism of the Soviet Union. During negotiations with the administration of U.S. President Franklin D. Roosevelt, prime minister Winston Churchill had promised to declare war 'within the hour' of a Japanese attack on the United States.On the 7th and 8th of December 1941, Japan attacked British and American territories in Southeast Asia and the Central Pacific with near-simultaneous offensives including an attack on the US fleet at Pearl Harbor.
Decision and communication
News of the attack on Pearl Harbor reached London first. Realising that Roosevelt would go through the formal process of asking the United States Congress for a declaration of war, Churchill began preparing to deliver Britain's own declaration of war immediately after Congress had formally declared war. After learning British territory had also been attacked, Churchill decided there was no need to wait for Congress to act and promptly summoned Mamoru Shigemitsu, Ambassador of Japan to the United Kingdom.Foreign Secretary Anthony Eden was in transit to Moscow at the time, so Churchill was in charge of the Foreign Office. Churchill instructed Shigemitsu to inform his government that a state of war existed between the two countries and drafted a letter to inform the ambassador of this. Of the letter, Churchill later wrote: "Some people did not like this ceremonial style. But after all when you have to kill a man it costs nothing to be polite."
The United Kingdom declared war on Japan nine hours before the United States. The earlier declaration by the British was due to their attacks on the colonies of Malaya, Singapore, and Hong Kong, and also due to the fact that they lacked the American constitutional tradition of requiring the consent of their own national legislature to declare war – the British cabinet could declare war without consulting Parliament, and could therefore act more quickly.