United Kingdom declaration of war on Japan
On 8 December 1941, the government of the United Kingdom declared war on the Empire of Japan, following the Japanese attacks on Malaya, Singapore and Hong Kong. The British Secretary of State for Foreign Affairs, Anthony Eden, was in transit to Moscow at the time, so Prime Minister Winston Churchill was in charge of the Foreign Office. 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 
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."