Only would have mattered if the US government was willing to make the sacrifices needed to work - like bargain on collective security; which they all but abandoned (though actually in the Treaty of Versailles) after 1920.
Anyone who thinks that the United States joining the League would have prevented future wars is seriously diluted. the United Nations is (potentially) more powerful than the League, and has it put an end to conflict? No.
Countries with armies larger than that of the United States were in the theaters of conflict and they prevented absolutely nothing.
The West divided up Czechoslovakia, they allowed the conquest of Austria, they sat there and twiddled their thumbs after Poland was attacked, even though they were officially at war.
Western Betrayal... even the aftermath of WWII has not reversed that trend.
The "conquest" of Austria. Hmmm....