The last country to join the Allied Powers during World War II was the United States, officially entering the war after the attack on Pearl Harbor in December 1941. In contrast, other countries like Great Britain and France had already declared war in 1939. This addition greatly strengthened the Allies' position against the Axis powers. ;
The last country to join the Allied Powers during World War II was the United States, after the attack on Pearl Harbor on December 7, 1941. This event marked the official entry of the U.S. into the war, enhancing the strength of the Allies significantly. Therefore, the correct answer is C. United States.
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