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Blitzkrieg into Poland
This invasion is important because it marked the start of World War II. Before the invasion Britain and France has signed acts to protect Poland, therefore triggering a much bigger war.
The invasion was led by Germany and Russia ( Soviet-Nazi Act ) as Germany invades fast and fast troops and air support while Russia comes in from the East. Germany and Russia had signed a non-aggression pact on August 23, 1939.