On 1 September 1939, Germany invaded Poland. Two days later, on 3 September 1939, after a British ultimatum to Germany to cease military operations was ignored, the Prime Minister of Britain, Neville Chamberlain, declared war on Germany.
World War 2 was fought between two groups of countries. On one side were the Axis Powers, including Germany, Italy and Japan. On the other side were the Allies. They included Britain, France, Australia, Canada, New Zealand, India, the Soviet Union, China and the United States of America.
Germany was ruled by Adolf Hitler and the Nazi Party. Hitler wanted Germany to control Europe.