What role did Winston Churchill play in WW ll?

The British Prime Minister.

Before Prime Minister he was First Lord of the Admiralty.

Chamberlain was Prime Minister but he resigned after the failure to protect Norway from German invasion and Churchill took over, even though as First Lord of the Admiralty, the Norwegian Campaign was largely under Churchill's control.

It was Churchill's use of the Navy around Norway that encouraged the Germans to attack, in the hope that the British could draw them into a trap.