Would the League of Nations have been successful if the US had joined?

I don’t think it would be much different, at least until the 1930s.Even if the US had joined the League, it’s hard to see changing its policy: for example, the Harding Administration was adamant they will not cancel the war debt acquired by the Entente. When the Allies defaulted on their Great War loans (still unpaid AFAIK), the reaction in the US was of anger: it’s hard to see the US eager to intervene and support the debtor nations during the crisis of the 1920s. Add the Great Depression and probably the US would just be a formal member of the League without much interest on direct intervention.

Soory i don’t know hope you get a good grade