The UN is a first step, or actually a second step, the League of Nations was the first step, towards an international framework to resolve differences with out war.
Since it has been formed, about a million people a year have died in wars on average. Sounds bad, but this is a huge improvement over the 50 years before it was formed when hundreds of millions were killed in wars.
Like every human endeavor, it is far from perfect, but it has had numerous successes, and all in all costs only a small fraction of what wars cost in the first half of the 20th century. If the current administration had gone through the UN instead of launching an unprovoked invasion of Iraq, perhaps another 100,000 lives would have been spared.

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“ The subjects of every state ought to contribute towards the support of the government, as nearly as possible, in proportion to their respective abilities; that is, in proportion to the revenue which they respectively enjoy under the protection of the state.”

Adam Smith , The Wealth of Nations 1776

"We have always known that heedless self-interest was bad morals; we know now that it is bad economics"
FDR's second Inaugural Address