When did Britain declare war on Germany?
British declaration of war in WW1
In WW1 Britain declared war on Germany on 4 August 1914 shortly after German troops entered Belgium.
British declaration of war in WW2
In WW2 Britain declared war on Germany on 3 September 1939.
The British Prime Minister, Neville Chamberlain, declared war on Germany on September 3, 1939, two days after the Nazi Germans invaded and attacked Poland. Obviously, he did so with the backing of Parliament.
3rd September 1939. It was a joint declaration of war by Britain and France.