What was first thing Hitler or the Germans did that was an important cause of Germany's defeat?

Germany's ultimate defeat resulted in a string of small failures: * It failed to win a decisive victory at Dunkirk, allowing 345,000 troops to escape. * It failed to win the Battle of the Atlantic, which meant that * It failed to win the Battle of Britain because it attacked cities instead of airfields and radar. * It invaded Russia without preparing for Winter War. * It declared war on the US. * It continued to use its faulty Enigma encryption system.

It stated the war. then it had two fronts.