Answer: there were a lot of basic reasons but the immediate reason was when Germany invaded Poland in September 1 1939.
Explanation: the basic causes were the great depression, Fascisim, Nazi party, when Japan invaded Manchuria, Spanish civil war, Versailles Treaty, the weakness of the UN, US policy of isolation, Britain and France appeasement. In 1938 the British france and Germany signed the Munich deal which allowed Germany to take parts of Czechoslovakia. and also in 1939 Germany and the USSR signed the Nazi-Soviet non aggression pact. but finally when Germany invaded Poland in September 1 1939, Britain and France declared war on Germany