the UN was founded by people who went through a horrendous war, (WWII)
and wanted to avoid another major world war. And many of the founders were farsighted Americans who knew that the next world war will be even more destructive.

ITs not perfect, but by allowing an international forum, and establishing legal rules for war, the UN has done what it was intended to do(preventing WW3). Right now the major threat to the UN is not corruption or ineffectiveness, it is only as effective as its members are willing to be, but rather the US and its neocon leadership which has stated that it wants to 'maintain military supremacy for all time.' THat leadership would rather scrap the UN and go back to the alliances and ententes (or the law of the jungle) of the 19th century - and we know that worked really well.

Its utter nonsense of course because ultimately it comes down to economics and within 30years the US economy will be eclipsed by CHinas
(as Goldman Sachs predicts) and the US dollar (as a world currency) will go the way of the pound sterling (the previous world currency)

You can also have the largest military in the world but lets face it, its not that useful in a country with a low tolerance of US casualties, and how effective is it when the largest military in the world cant maintain order in a tiny country the size of Iraq after 3 1/2 years.
Not to mention the cost - of the war to the US economy estimated to be between 1.5-2 trillion (by the George Stiglitz /World Bank economist)
add to that the debt created by George W (bigger in his term than ALL the debt by the previous presidents from George Washington combined)