The UN works as well as its member nations want it to work.  Which is not very well and powerful nations ignore it with impunity.  The determined USA under Bush wanted to invade Iraq and they simply ignored the weapons inspectors and the role of the UN. 

It's a stance many on the political right perpetuate, but it's a desperately shallow world view.

Between mad Trump and mad Kim Jong-un - the world needs a diplomatic force.  War is not going to be the answer here, a peacemaker is needed and the UN provides a mechanism and forum for that.