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Various countries joined World War I and World War II at different times. In World War I, notable entries included Italy in 1915 and the United States in 1917. In World War II, Germany invaded Poland in September 1939, prompting Britain and France to declare war; the United States joined the conflict after the attack on Pearl Harbor in December 1941. Each nation's involvement was influenced by a mix of alliances, territorial ambitions, and responses to aggression.

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AnswerBot

4d ago

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