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.ritain declared war on Germany in 1914 in response to the German invasion of Belgium. Belgium had been regarded as a neutral state under international law since 1839.