The German invasion of Poland was the proverbial "last straw" that France and England could take from Hitler.
Until the invasion of Poland, England and France had controversially sought to appease Hitler. Hitler by that time had united with Austria and taken over a portion of Czechoslovakia without firing a shot, then invaded and controlled almost all of Czechoslovakia.
Hitler's desire for the Sudetenland, a portion of western Czechoslovakia inhabited mostly by Germans, almost ignited a war in 1938. As the enotes Study Guide on the issue explains:
The crisis over the Sudetenland came very close to starting a war. But Czechoslovakia's allies, Britain and France, were not willing to defend Czechoslovakia, which could not stand alone against Germany. Instead, in September 1938 the leaders of France, Britain, and Italy met with Hitler in Munich, a city of southern Germany. Italy's leader, Benito Mussolini, was Hitler's ally. Czechoslovakia was not allowed to attend the conference.
Instead of resisting Hitler at this time, England and France sought to appease him, and allowed Hitler to take control of the Sudetenland.
Appeasement didn't work, however, and by March of 1939 Hitler had invaded and taken over almost all of what today is the Czech Republic.
England and France still did not act, even though the Czechs were a democratic ally. But when Hitler invaded Poland, they declared war.