Would a US-involved League of Nations have prevented World War II?

[The Mikado]

The moment the League's uselessness became obvious was in 1931 when Japan invaded Manchuria. The League condemned it, Japan responded by withdrawing from the League and...nothing happened.

If the USA had been in the League as well, I see no reason to think these events would've gone differently.

Ditto five years later when Italy invaded Ethiopia despite the League Charter saying that the League would protect the independence and territorial integrity of its members. No one wanted a war over it so the League did nothing.

The League of Nations could do one thing reasonably well: arbitrate disputes between countries that don't REALLY want to fight over the dispute. Their greatest success story was adjudicating whether the Aland Islands should be in Finland or Sweden. Neither Finland nor Sweden had any interest at all in fighting a war over the islands, so handing them over to adjudication was a reasonable situation, and the eventual result (Finland gets the islands but can't make any move to suppress Swedish language or culture) was acceptable to all parties.

These circumstances weren't present with the Manchuria crisis or the later Ethiopia crisis. Japan and Italy WANTED wars.