Was the United Nations effective in keeping the peace?

Depending on the mission criteria you set for the UN, I'd say yes they have.

Bear in mind that the UN is technically unarmed, poorly funded, and has no truly enforceable mandate. As such, you can't expect total world peace as a result of UN operations.

However, within the scope of what the UN actually can do, I think they get a passing grade.

Before leaping to an alternate answer, think for a moment and ask yourself (or others!) what the UN could have done that they failed to do.