The US was not a member of the League of Nations. It did sit in on conferences and participate to a degree. It did nothing effective following the Mukden incident and the conquest of Manchuria.
https://history.state.gov/milestones/1921-1936/mukden-incident
The United States and other western powers were at a loss on how to respond to the rapidly developing crisis. Even as the Japanese moved far from the original site of the “attack” at Mukden to bomb the city of Jinzhou (Chinchow), there was little sense that U.S. interests in the area were anywhere near profound enough to make military intervention necessary or desirable. Given the 1930s worldwide depression, there was little support for economic sanctions to punish the Japanese. Instead, the United States sat in on League of Nations council meetings for the first time to try to convince the League to enforce the Kellogg-Briand Pact, which both Japan and China had signed. Appeals based on the pact, however, proved ineffective.
True enough - but the US was not the world power it became later and the American people could not see that aggression in China could affect the US and the American people were still reeling from the expense and combat losses of the First World War.
Despite this - and we were a representative government - we nonetheless came to the support of China, despite the enormous distances, rugged and trackless terrain, the absolute lack of fuel, munitions, electricity, modern engineering equipment or an educated workforce, and many other large problems dealing with China and its various governments. We overcame all of that to build the infrastructure needed to support the air campaign against Japan and support Chinese combat operations in China - all while fighting a desperate and bloody war across the Pacific against Japan.
Anyone discussing the US role in China before and during WWII and then listening to you would think that you're just a mouthpiece for revisionist Chinese propaganda - right?
It did nothing effective following the Mukden incident and the conquest of Manchuria.
Neither did your goodbuddies Josef Stalin and the Soviet Union, who were right next door and more interests there than we did.