Japan and the U. S were once allies during WWI. What was the main reason that
changed that?

The main reason is as a result of their competing interests in Chinese markets and Asian natural resuources

Explanation:

Japan attacked the American Navy Base at Pearl Harbour , Hawaii on December 7 , 1941 . In response , The US declared war on Japan Japans allies includding Nazi and Germany declared war on the United States few days after the attack bringing the US into World War II.

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a. men

and during later times male slaves as well