World War II is widely considered to have begun when Germany invaded Poland on 1 September 1939.

What invasion started WWII?

  • Germany launched a massive surprise attack on Poland on 1 September 1939, using a fast-moving tactic later called Blitzkrieg.
  • In response, Britain and France declared war on Germany on 3 September 1939, marking the formal start of World War II in Europe.

Why not Austria or Czechoslovakia?

  • Germany had already annexed Austria in March 1938 (the Anschluss) and dismantled Czechoslovakia in 1938–1939, but these actions, while aggressive, did not trigger a general European war.
  • The invasion of Poland crossed a clear red line set by Britain and France, which had guaranteed Polish security, so this invasion is treated as the event that started World War II.

Quick quiz-style takeaway

If you see a multiple-choice question such as:

“World War II began with German invasion of:
A. Poland
B. Belgium
C. Austria
D. Czechoslovakia”

The correct answer is: Poland (A).

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