How did the united states attempt to weaken japan before world war ii?

By imposing economic sanctions (trade restrictions) against Japan.

Japan's imperialist aggression into Chinese territory was met by the US with a series of trade restrictions.  A 1911 trade agreement between the nations was canceled by the US in 1939.  The Export Control Act of 1940 limited trade to Japan even further.  In July, 1941, the US froze all Japanese financial assets in US markets, and in August, 1941, placed an embargo on oil and gasoline exports to Japan.  These actions were seen by Japan as acts of war and they retaliated with the attack on Pearl Harbor in December, 1941.