Why did nazi invasion of poland start world war 2

The nazi’s invaded Poland with the help of Russia. During the beginning of WWII, Hitler formed an alliance with Stalin, the leader of Russia at the time. They used a tactic where they attacked Poland from both the East and the West. This started the world war because the countries who had alliances with Germany and Russia helped them fight their side of the war, and other countries who had an alliance with Poland joined in as well. Eventually, the war progressed into a world war because of the amount of countries involved.
Hope this helped!!

Because of the alliances made at the time