Who was the British Prime Minister when World War II began in September 1939?

The British Prime Minister when World War II began in September 1939 was Neville Chamberlain.

Chamberlain had been Prime Minister since May 1937 and was still in office when Germany invaded Poland on September 1, 1939. Britain and France had pledged to support Polish independence, and after Germany refused to withdraw, Chamberlain announced that the United Kingdom was at war with Germany in a radio broadcast on September 3.

His government led Britain through the first eight months of the war. Chamberlain's earlier policy of appeasement, especially the Munich Agreement of September 1938, had sought to avoid war by allowing Germany to annex the Sudetenland. The policy was initially popular with many Britons but became deeply discredited after Germany occupied the rest of Czechoslovakia in 1939.

Chamberlain resigned on May 10, 1940, after criticism of his wartime leadership and the failed Norwegian campaign. Winston Churchill succeeded him that same day. Stanley Baldwin was the preceding prime minister, while Clement Attlee was Labour leader and Churchill became prime minister during the war—not at its beginning.