Violence can thrive without a cycle. I'm not saying what America has done in Iraq was justified or correct, but retreat will make things far worse than they are now.
We broke, we have to fix it. The surrounding countries(Iran......) would take advantage of a young Iraqi government and its frail armed forces to steal their oil recources.
And ya you'll probably say "that's what George Bush is doing."
I'm not a military expert, but retreat doesn't sound like much of a strategy either.