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.
On 7/8 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. During negotiations with the administration of U.S. President Franklin D. Roosevelt, British Prime Minister Winston Churchill had promised to declare war "within the hour" of a Japanese attack on the United States.
News of the attack on Pearl Harbor reached London first. Realizing that President Roosevelt would go through the formal process of asking the United States Congress for a declaration of war, Churchill began making preparations to deliver the UK's own declaration of war immediately after Congress had formally declared war. After learning British territory had also been attacked, the British Prime Minister decided there was no need to wait for Congress to act and promptly summoned the Japanese Ambassador.
The British Secretary of State for Foreign Affairs, Anthony Eden, was in transit to Moscow at the time, so the Prime Minister, Winston Churchill, was in charge of the Foreign Office. Churchill instructed the British Ambassador to Japan to inform the Japanese government that a state of war exists between the two countries and drafted a letter to the Ambassador of Japan to the United Kingdom to inform him 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 U.S. did. The earlier declaration by the UK was due to the Japanese attacks on the British colonies of Malaya, Singapore, and Hong Kong, and also due to the fact that the British did not have the American constitutional tradition of requiring the consent of its own national legislature to declare war - the British cabinet could declare war without consulting Parliament, and therefore could act more quickly. Churchill had previously committed to declaring war "within the hour" of a Japanese attack on the United States.
Text of Churchill's letter
The text of his letter to the Japanese Ambassador was as follows:
On the evening of December 7th His Majesty's Government in the United Kingdom learned that Japanese forces without previous warning either in the form of a declaration of war or of an ultimatum with a conditional declaration of war had attempted a landing on the coast of Malaya and bombed Singapore and Hong Kong.
In view of these wanton acts of unprovoked aggression committed in flagrant violation of International Law and particularly of Article I of the Third Hague Convention relative to the opening of hostilities, to which both Japan and the United Kingdom are parties, His Majesty's Ambassador at Tokyo has been instructed to inform the Imperial Japanese Government in the name of His Majesty's Government in the United Kingdom that a state of war exists between our two countries.
I have the honour to be, with high consideration,
- Your obedient servant,
- Winston S. Churchill
Winston S. Churchill: The Second World War (vol.3): the Grand Alliance. (1950) ISBN 0-395-41057-6