The German high command formed an army despite being forbidden by the Treaty of Versailles. The troops entered Poland on September 1, 1939. Germany justified this act of aggression by saying they were rescuing German civilians from illegal enslavement by the Polish government.
Germany’s expansion into Poland was to be answered with a declaration of war from both France and Great Britain. They had agreed to maintain the established borders of Poland and with Germany aggressively crossing the Polish border, these countries had to act. The Soviet Union had a pact with Germany and also invaded Poland with the end goal of dividing the country with Germany. Each of these countries justified their actions as appropriate and war began.