Confused

The thing that I am having a hard time understanding is the whole Shiite and Sunni thing. I heard on the news today that Hezbollah insurgents are coming and fighting our troops in Iraq. Hezbollah are Shiite and so is the main population of Iraq. The Sunnis have said it is all right to kill the Shiites, so why aren't the Hezbollah fighting the Sunnis instead of the Americans? I do know the answer and that is because the USA is the great Satan. I also wonder if the insurgents understand that if they would knock their shit off and let things mellow out for awhile then the US would leave and they could try and do whatever they feel the need to do. I don't know, that's just my thoughts on that.