In an oil rich middle eastern country, two rivals are waging a civil war in a fight for power.  US officials meet and decide to support tyrant A because they think he will be easier to deal with in the long run and will have a more stabilizing effect on the region.  They supply him with arms.  A defeats B and then goes and kills millions of his supporters.  Many of them women and children.  The US gets blamed for interfering.

 Or

US officials meet and decide to support tyrant B because they think he
will be easier to deal with in the long run and will have a more stabilizing effect on the region.  They supply him with arms.  B defeats
A and then goes and kills millions of his supporters.  Many of them
women and children.  The US gets blamed for interfering.

or

In an oil rich middle eastern country, two rivals are waging a civil
war in a fight for power.  US officials meet and decide to do nothing.  A defeats B and then goes and kills millions of his
supporters.  Many of them women and children.  The US gets blamed for NOT interfering.

or

In an oil rich middle eastern country, two rivals are waging a civil
war in a fight for power.  US officials meet and decide to do nothing.  B defeats A and then goes and kills millions of his
supporters.  Many of them women and children.  The US gets blamed for NOT interfering.

Notice that tyrant A and B rarely get the press for their horrible crimes against humanity.  They fire the guns, blow up the bombs that kill the people, and the US gets to shoulder the blame. 

It's a small world after all.  And it's getting smaller.  The US must do something, along with the international community, to stop the thugs of the world.  N. Korea is a good case in point.  You people feel safe with these thugs having nuclear arms?  Forget who else has them.  You ok with N. Korea getting nuclear bombs and long range missles? 

If you say no to that question, then what's to be done?  Diplomacy has the word "dip" in it.  You'd be a dip if you thought diplomacy would work.  These folks don't think like you think.  

Many of the arguments one here assume a far less complex world than actually exists.