Soldier walks into a 7-11 with an M16.

Bad guy in the store wants a weapon upgrade.

Bad guy moves behind the soldier and caps him with a .22 pistol and takes his M16.

Bad guy might be able to take out 2 or 3 unsuspecting soldiers. If bad guy has a friend there who also has a .22 pistol, it becomes very easy.

You can probably point to counter-examples, but I think you're exaggerating the "being able to take weapon away" aspect of the entire thing.
I'm talking about walking up behind an unsuspecting soldier and putting a .22 pistol to his head and killing him. I'm not talking about a firefight.

If the U.N. or the U.S. wants to have an "international police force" of some sort, fine - train one that's designed to do that, don't coopt member nations' armies into jobs they're poorly equipped and not trained to do. If the U.N. or the U.S. feels it wants to go to war, then do so, don't go about it half-baked.
Yup. "Peace-keeping" is not something the military can do. At best they can stop the bad guys from doing anything overt until they leave. Only a local police force can keep the peace.