The last country to join the Allied Powers during World War II was the United States. Here's why:
1. **United States**: The United States officially entered the war on December 8, 1941, following the Japanese attack on Pearl Harbor. This event led to the United States declaring war on Japan and, subsequently, on Germany and Italy, aligning itself with the Allied Powers.
By joining the Allies, the United States brought significant military, industrial, and economic resources to the alliance, playing a crucial role in the eventual victory over the Axis Powers.
