UN or EU?

Hey all, I hope your weekend is going well. All the election frenzy has brought to mind a question about the United States. Why do we continue to belong to the UN, and does European countries even care anymore about NATO with the existence of the EU.
America has always assumed a role as top dog in most occasions. We have deemed number one in many respects. Often politicians approached the United Nations assembly to seek their support if any action regarding a foreign policy matter needed to be address. They have never really as a whole cared though. When former Secretary of State Colin Powell made his famous speech regarding Iraq many top European countries turned their backs. Britain remained loyal along with some other smaller countries. The instances when we’ve chosen to stand by UN policies have resulted in international disasters. The refusal to recognize the massive massacres in Sudan as a genocide made impossible for American aid as UN Human Rights Council didn’t see it as a genocide. It didn’t help when the Iranian leader Ahmadinejad got to speak in New York in 2007. No wonder European countries formed the EU.
I am wondering though if the lack of an identity of the UN along with the current political strength of the EU presents a problem for America in international politics. The European Union as noted in Encyclopedia Britannica was founded in 1999. A body that helps decides many of the current political and social policies enforced across Europe. Many European countries as reported by the BBC hope to add military strength to the EU’s portfolio. If they focus all their effort in building a strong governing body the UN will continue to loose power. The United States will continue to pay its fees along with the constant harassment of its policies, and in the mean time European counterparts focus more on increasing the power of the EU.
The main question in this rant lies in whether the United States should remain as loyal to the United Nation. What alliance can the United States really trust in case disaster ever struck? I know this is quite gloomy, but it’s a valid thought to contemplate. James.