If Assad is using Sarin gas on innocent people, I think it's the world's responsibility, Russia included, to stop him. It's not just the responsibility of the U.S.
And as far as Afganistan goes, nobody "wins" in Afghanistan. The Russians learned that. The English learned it. If there is one lesson of history that should have been pounded into peoples' brains by now, it's the fact that nobody "wins" in Afghanistan. It's like attacking Russia in the winter. You don't do it. The U.S. target and goals in Afghanistan should have been extremely limited: Take out Al Queda. Once we killed bin Laden, we should have immediately pulled out.
So why stay in Afghanistan? Or Korea? Or Germany? Or all the other places where we spend billions of dollars to keep a military presence?