Because we invaded their country
Its not rocket science, Tom. We set this whole thing in motion. Now that we're there we have to succeed and finish the job. But in a country of over 22 million people, it only takes a small percentage of "insurgents" to wreak havoc. I predicted this going in. All one needed to do is reflect upon the Palestinian conflict to the south and realize that Iraq would replicate the experience with a pool of potentially perpetual insurgents 22 times large than in Gaza and the West Bank.