When there is conflict in the world that many civilized countries agree needs to be dealt with it is not fair that one or two nations wind up sending all the troops into combat. A standing army of the UN would be composed of all its members' troops and that would make things fair.
1 - The U.N. could grow in influence and power and begin to dictate to the world how things will be and who can stop a force with international troops?!
2 - Who would lead the troops? In other words an Army belongs to a governing body and if the U.N has an army then they would be a bonefide world army which in turn means they must be in place to protect and fight violations to laws. That begs the question who laws, the UN charter? And if the leader of the UN was another Hitler because power corrupts and absolute power corrupts absolutely!
3 - What will happen to national sovereignty? GONE, that what because eventually all the drastic changes will come about with any input from the people.
4- Will the UN be a force or democratic power, a republic or communist? This must be understood first and since the UN is highly in favor of socialism then my guess the army would uphold, defend and even implement communism by FORCE!!
SO, NO WAY NO HOW should the UN have an army!!!
I do not believe it will be beneficial for the UN to have its own specific standing army. While the UN is a coalition of multiple national representatives, I fear there will be a sort of power debate over who actually controls the army. Now, if thorough regulations can be formulated and adhered to, then the possibility of an army might become possible.