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 â¦ 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.reat Britain declared war on Germany and did not come to the rescue of Poland as they had intended to do. They did try to help France defeat the Nazi Germans. It was a failur â¦ e. There was a period of about 4 months duration (I believe it was that long) that was called the Phony War.