The previous thoughts were quite accurate in that the German invasion of Poland ended up being the proverbial straw that broke the camel's back. The interesting thing was that leaders were actually surprised that Hitler did it. Seeing that his pattern was to subsume most of the continent, it is not all that surprising. At the same time, I would say that the significance of the event is actually more in line with Hitler's own philosophy. The concentration of Jewish people in Poland was significant. In annexing Poland, the atrocities of Hitler and the Nazis ended up becoming greater in scope and number. It is not a surprise that most of the death camps the Nazis operated were in Poland and a large number of its Jewish citizens were killed. While Jewish individuals were present all over Europe, the concentration in Poland helped to make it such a target for Hitler.
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.
Well... the German invasion of Poland in September of 1939 is one of the most important actions ever taken by a country. The significance of this invasion is hard to overstate since this invasion is what started World War II.
The British and French had decided that Hitler had taken too much territory in Europe and were determined to stop him. They guaranteed the borders of Poland but Hitler did not believe they would go to war on Poland's behalf. He was wrong.
When Hitler invaded Poland, Germany and France declared war and WWII had begun.