What action by japan did the united states firmly oppose?

It’s expansion in Asia

Explanation:

The United States had a strong trade agreement and it was mainly responsible for supplying items such as metal, oil, etc to Japan. This is what Japan used in its war against China and United States didn’t like it.

Due to the legality of the trade agreement between Tokyo and Japan in 1911, the American government told the Japanese in 1939 of its intentions to end the trade agreement. It then stopped supplying materials to Japan due to its expansion in Asia which was a potential threat to its quest for world power also.