The Churchill war ministry was the United Kingdom's unity coalition government for most of the Second World War from 10 May 1940 to 23 May 1945. It was led by Winston Churchill, who was appointed prime minister of the United Kingdom by King George VI following the resignation of Neville Chamberlain in the aftermath of the Norway Debate.

At the outset, Churchill formed a five-man war cabinet which included Chamberlain as Lord President of the Council, Clement Attlee as Lord Privy Seal and later as Deputy Prime Minister, Viscount Halifax as Foreign Secretary, and Arthur Greenwood as a minister without portfolio. Although the original war cabinet was limited to five members, in practice they were augmented by the service chiefs and ministers who attended the majority of meetings. The cabinet changed in size and membership as the war progressed but there were significant additions later in 1940 when it was increased to eight after Churchill, Attlee, and Greenwood were joined by Ernest Bevin as Minister of Labour and National Service; Anthony Eden as Foreign Secretary, replacing Halifax who was sent to Washington as Ambassador to the United States; Lord Beaverbrook as Minister of Aircraft Production; Sir Kingsley Wood as Chancellor of the Exchequer; and Sir John Anderson as Lord President of the Council, replacing Chamberlain who died in November (Anderson later became chancellor following Wood's death in September 1943).

The coalition was dissolved in May 1945, following the final defeat of Germany, when the Labour Party decided to withdraw in order to prepare for a general election. Churchill, who was the leader of the Conservative Party, was asked by the King to form a new, essentially Conservative, government. It was known as the Churchill caretaker ministry and managed the country's affairs until the completion of the general election on 26 July that year.

The 1935 general election had resulted in a Conservative victory with a substantial majority and Stanley Baldwin became prime minister. In May 1937, Baldwin retired and was succeeded by Neville Chamberlain who continued Baldwin's foreign policy of appeasement in the face of German, Italian and Japanese aggression. Having signed the Munich Agreement with Adolf Hitler in 1938, Chamberlain became alarmed by the dictator's continuing aggression and, in March 1939, signed the Anglo-Polish military alliance which supposedly guaranteed British support for Poland if attacked. Chamberlain issued the declaration of war against Germany on 3 September 1939 and formed a war cabinet which included Winston Churchill (out of office since June 1929) as First Lord of the Admiralty.

Dissatisfaction with Chamberlain's leadership became widespread in the spring of 1940 after Germany successfully invaded Norway. In response, the House of Commons held the Norway Debate from 7 to 9 May. At the end of the second day, the Labour opposition forced a division which was in effect a motion of no confidence in Chamberlain. The government's majority of 213 was reduced to 81, still a victory but nevertheless a shattering blow for Chamberlain.

On Thursday 9 May, Chamberlain attempted to form a National Coalition government. In talks at Downing Street with Viscount Halifax and Churchill, he indicated that he was quite ready to resign if that was necessary for Labour to enter such a government. Labour Party leader Clement Attlee and his deputy Arthur Greenwood then joined the meeting, and when asked, they indicated that they must first consult Labour's National Executive Committee (which at that moment was in Bournemouth to prepare for the party's annual conference), but it was unlikely they could serve in a government led by Chamberlain; they probably would be able to serve under some other Conservative.

After Attlee and Greenwood left, Chamberlain asked whom he should recommend to the King as his successor. The version of events given by Churchill is that Chamberlain's preference for Halifax was obvious (Churchill implies that the spat between Churchill and the Labour benches the previous night had something to do with that); there was a long silence which Halifax eventually broke by saying he did not believe he could lead the government effectively as a member of the House of Lords instead of the House of Commons. Churchill's version has an incorrect date, and he fails to mention the presence of David Margesson, the government Chief Whip.

Halifax's account omits the dramatic pause and gives an additional reason: "PM said I was the man mentioned as most acceptable. I said it would be hopeless position. If I was not in charge of the war (operations) and if I didn't lead in the House, I should be a cypher. I thought Winston was a better choice. Winston did not demur." According to Halifax, Margesson then confirmed that the House of Commons had been veering to Churchill.^{[citation needed]}