When Japan invaded and captured Manchuria, China turned to the League of Nations to help contain Japanese Aggresion towards China.
The League of Nations passed a resolution telling Japan to withdraw troops which they hoped would work. Japan did not like this resolution and insisted on direct negotiations with China. When this failed, Japan left the League of Nations entirely.
The League of Nations established the Lytton commission to investigate the extent of the damage to the Japan railways and if it was intentionally done by China. The commission took months to arrive and several months to come up with a report.
The US and its government decided to stay out of Japanese affairs because the US did not have a strong naval presence in the Pacific and America was dealing with its own Great Depression. American economic interests were not strong enough to inspire intervention. They also did not want to risk trades with Japan by intervening.
(1937-1938) Roosevelt provided financial aid to China but did not want to get politically involved, even rejecting British appeals for US participation. Some argue that the US partially supported Japan's war effort by continuing to trade with them.
USA became more aggressive on Japan after 1938. The US also started supplying oil to China. USA became more watchful of Japan's actions after Japan signed a treaty stating the alliance of Japan, Germany, and Italy.