The UN is corrupt

That does not make any sense. If the U.S. LEAVES the UN then the terrorists will be even MORE afraid due to the fact that now the U.S. could do what it wanted instead of having to ask the pussy corrupt UN.

I would seriously doubt that leaving the UN would frighten the terrorists. You have to think from their mindset. These people are crazy. Lunatics. More than likely, they don't care about anything other than accomplishing whatever it is they set out to do. They don't care about death, about family, about morality, most probably, and when it's malice that you are full of, then that is a bad combination. Widthdrawing from the UN won't hinder their attacks on us.

WOW! POLITICIANS ARE CORRUPT?! MY WORLD IS SHATTERED!

You just have to choose the less corrupt of the two. Thus, I take the UN.

If anything, it would make us more vulnerable to attack since we won't have many allies that would be willing to retaliate.

Kuja that did not make much sense considering the UN itself is corrupt...

We only have two good allies as it stands now, and they aren't going to leave our side. So I really don't know what else we could lose. As for us leaving the UN we don't even listen to them so whats the difference if we weren't there.