As conservatives used to be fond of saying, the US should not be the policeman to the world.

I agree. But I believe that the world does need a policeman. E.g., Serbia, Rwanda, Uganda, Afghanistan, Iraq, Haiti.

So, who?

The UN would be the answer, if it were capable of bilateral force.

I believe that the answer is an expanded, empowered NATO.

Ultimately, nations will have to give up sovereignty in order to solve global problems.

Perhaps a confederation would be a good approach.

Someone needs to write a global constitution.