The excerpts below are from a PDF by The National Intelligence Estimate, where it was originally published in Jan. 2007.
Iraq’s neighbors influence, and are influenced by, events within Iraq, but the involvement of these outside actors is not likely to be a major driver of violence or the prospects for stability because of the self-sustaining character of Iraq’s internal sectarian dynamics.
Nonetheless, Iranian lethal support for select groups of Iraqi Shia militants clearly intensifies the conflict in Iraq. Syria continues to provide safehaven for expatriate Iraqi Bathists and to take less than adequate measures to stop the flow of foreign jihadists into Iraq.
...
If such a rapid withdrawal were to take place, we judge that the ISF would be unlikely to survive as a non-sectarian national institution; neighboring countries invited by Iraqi factions or unilaterally might intervene openly in the conflict;