The League of Nations could claim as an excuse that it really had no commitment of force from members supporting it. The U.N. can make no similar claim. It has the force necessary but has no sincere interest or intention of using it. Why anyone would bother to establish, train, and finance a military force and not use it demonstrates its faithlessness.
The League of Nations was completely unsuccessful in preventing or influencing the second world war, watching from the sidelines while Hitler grabbed Austria, Czechoslovakia, and finally Poland.
The U.N. has been equally incompetent in preventing or stopping any war that I am aware of. It prides itself on discussing (interminably) possible resolutions to problems but always ends up "observing", while Tutsi and Hutu slaughtered each other, and was equally circumspect not to intrude in the killings in Bosnia and Somalia. During the past 50 years,only the U.S.(supported thankfully by Great Britain) has made any timely and substantive effort to stop the killings in South Korea, Kuwait, and Kosovo.
An obvious and necessary question is why the U.N. is so incapable and ineffective. This is not a difficult question. The truth is that a majority of the member countries constituting the General Assembly simply want to use the U.N. to promote their own interests which, unfortunately, consist mainly of actions designed to undermine the efforts of other member states to accomplish anything of substance. This practice derives from the jealousy and ill-will of poorer countries toward rich countries, and their attempts to hinder whatever the richer countries wish to accomplish. The governments of these countries are unable to sufficiently provide a successful working economy for their own citizens, and attempt to deflect blame for this failure toward other countries more successful in providing for their citizens. All the hot air generated in the General Assembly is designed to do just that(and nothing more).
George Bush is criticized for not paying sufficient attention to this body but it is to his credit that, after attempting to get consensus in the U.N., he had the courage to act, much like Clinton had in Kosovo. You have to give him credit for going ahead and doing what he felt was his duty as president and ignoring the temporizing attempted by other Assembly countries intent on pursuing their own(oil) interests with little genuine concern for Iraq. Only history will tell whether President Bush was right or wrong in doing so, and he will eventually have to accept the consequences of what he has done.