How did the invasion of poland lead to ww2?

Despite looking at Hitler with suspicious eyes, the French and British officials didn't do anything when Germany annexed part of the Czech land. Instead, the French and the British thought that that would satisfy Hitler's appetites and that he will not dare to make anymore troubles.

They were wrong though. Hitler ordered an invasion of the neighboring Poland, though he was saying that it was not an invasion but it was a defensive action because of the Polish threat toward his country. The Polish though didn't had any ambitions toward Germany, neither they did something to them, nor did they had the military power for such an action. Also, Hitler saw Poland as a place for living of the Germans, and he wanted to exterminate the Polish people.

That triggered the alarm and France and Great Britain declared war on Germany because of its actions and breaking of the treaty, thus the World War II was officially initiated.