Hillary and protection money, er, campaign contributions

The League of Nations fell into irrelevance because a) the US declined to join, and b) because Europe remained largely an armed camp of country's with colonial empires. If only as a vehicle for involvement by countries carved out of those empires, the UN, as toothless as it can be, remains relevant, and its primary function was never military -- which most people on the crazy-ass right will tell you if you ask them about American soldiers serving under foreign command.