The UN should not have a standing army. As an international organization, member nation states will often have differing views on when a standing army should and should not be mobilized. As evidenced in recent years, member nations, specifically those on the Security Council, often disagree on what the UN's policy should be in world affairs. As an American, I do not want an Army run by the UN deployed onto American soil, and the perception of a nations denial of and intrusion of sovereignty would cause more international tension than it would ever relieve.
Right now the UN is there for negotiations, there is no reason for it to have a standing army. It is not a typical ruling body and until it becomes on, which it may very well do sometime in the future as globalization increases, it does not really need an army.
The United Nations by definition is a group of nations that pool their resources for the good of the world at large. If the U.N. had a standing army, who would be in charge of it? Who would get to say what it could and could not do? As it is, individual countries can lend their forces to the U.N., and that is as it should be.
No, the U.N. should not have a standing army.
The U.N. is, quite frankly, a joke. The best example of this in their inability to intervene in the genocide in Rwanda in 1994. Despite the violence that surrounded them, the U.N. forces were not allowed to fire a single shot or provide aid to any of the citizens. How in the world does that qualify as a "peace keeping" force?
The U.N. needs to be reformed as an establishment that actually has some power. Then, and only then, should they consider having a standing army.