The United Nations does not fulfil it's primary purpose of maintaining world peace. It has failed as a moderator between the arabs and the Israelis. It has stood by shamefully as the arab countries continually engage in anti-semitic terrorism and aggression against the soverign state despite having stationed peacekeepers in the region for generations. The UN has passed countless resolutions against Israel when it defends itself. Warfare continuous to bring millions of deaths across every continent. The UN has stood by and allowed preventable genocides to take places in Myanmar, Rwanda and Bosnia, even having peacekeepers on the ground at the latter two countries who did nothing as per their orders. The UN has a liberal political agenda which it pushes on soverign states when it interferes in their affairs. Such as when it accused Ireland of human rights violations for having a constitutional right to life of the unborn. Why does the UN lobby for abortion in Ireland while doing nothing to curb actual human rights violations in China, Saudi Arabia, Iran and North Korea to name a few.
The United States continually steps in to fill the breaches the UN is not willing to and does for more the promotion of free will and world peace.
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.