Which German attack provoked Britain to go to war? 

The Germans attacked Belgium, which in turn infuriated the British. The British who weren’t actively involved in the war before this were triggered by this attack. As a result the British decided to go to war against the Germans. It was in 1939 that that Britain and France declared war against the Germans.

They say that the battle of Britain was a form of a military campaign for the World War II. The British won after the turmoil. Wars can start on provocative attacks on different cities. Sometimes nations attack allies on account of which countries have to step in to fight for them.