United Nations is a key to world peace and collective military alliance.
For world peace, The UN kept the US, France, and other members from declaring war on each other.
Military alliance, on the other hand, depends on it. I mean if we can't be diplomatically related to a foreign government, what's the point of being allies?
I don't understand why we would. It's a force for the better, aiming to make the world a better place and make sure past evils don't occur in the future. A quote from their page:
"The United Nations was created more than 60 years ago to save succeeding generations from war, protect human rights, establish conditions for justice, and promote social progress and better standards of life. These fundamental objectives remain as critical as ever."
To the question 'Has the United Nations failed?', well I'm not sure what it's referring to. Naturally the UN could be better, but nothing is perfect.