Actually, Japan invaded China in 1931 starting with the invasion and conquest of Manchuria, turning it into the puppet state of Manchukuo. The Japanese invaded Korea starting in 1910.
US direct support started in 1937. As early as 1915 the US was already growing opposed to Japanese expansion but saw Japan as a more important market than China. When WW 1 ended, the US wasn't thrilled that Japan took concessions, like German colonial territory in China, and retained control of areas they had occupied at the end of that war. In the 20's the US tried to use tariffs to influence Japanese actions in China. But it was in 1937 that the US started massive aid to China and openly opposed Japanese expansion.
An early example of US aid would be the establishment of CAMCO in Hangzhou. This US-Chinese company was set up to assemble US manufactured (shipped in kit form) military aircraft for China. Later, it did depot level repairs for AVG (Flying Tigers) aircraft before ending up having to relocate to India once the Pacific War started.