The UN isnt as bad as the league was, the main problem with the UN is the right to veto by the security council, with the league you had japan droping out, america and the USSR not in, and then britain and france going against the league.
I just mean in this particular case, with manchuria, not the greater reasons for the leagues general failiure.
I would of said that the depression was the reason, every country just started to look for their own economies more, so they didnt embargo japan like the league wanted.