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the invasion of poland
it triggered the declaration of war by Britain and France against Germany -
great Britain and France declare War on Nazi Germany
signifying a united front against German aggression and preventing Hitler from easily dominating Europe without significant opposition -
the invasion of belgium, luxembourg, netherlands & france
it allowed them to quickly bypass the heavily fortified French Maginot Line, gain access to vital ports on the English Channel, and effectively knock France out of the war, paving the way for German dominance over Western Europe.