What would you consider to be the military Arm of the United Nations?

The United States, as well as the financial arm, political arm...without the US there is no UN

The United States

Well, there is no official military 'arm' of the UN. All military decisions are made within the UN Security Council, and then the logistics of planning and military commitment are worked out among member states. So if you had to mention an institution that involves military action, it would be the Security Council.

UN Peacekeeping Forces are drawn from member nations trained military. They usually try to draw on nations that are close to the affected area, but this can create political problems between the nations, so often the Peacekeepers come from distant lands.

The UN, per se, does not have a standing military "arm" of its own.

The UN Peacekeeping Troops and the Security Council.

The secret US Death Squads operating in Mexico, Central America, South America, Middle east and Africa

I consider anything done in the name of the UN useless and ineffective.

the peace keepers but they are ineffective and have no force behind their missions, they seem to spend more time getting away then helping

A bunch of monkeys at the zoo flinging their own feces at one another.

Whatever international force they cobble together. Generally its not very effective.