I think three things:

1) The US should never have gone in
2) Since they ARE in, they'd best not leave until there is a government in place that accommodates all major groups in Iraq: Sunni, Shiite and Kurd--or people could likely expect to see civil war and a slaughter akin to that which followed the US 'withdrawal' from Vietnam
3) The UN should be asked to send peace-keeping troops for a while, until it's clear that the new government will be able to govern