Juan Cole explains why Hillary Clinton's plan to indefinitely keep a small portion of the U.S. military in Iraq is be a bad idea. I recommend the entire article. Excerpt:

"...There are no Iranian units in Iraq. There are no Iranian prisoners to speak of in US custody in Iraq, even though 12,000 prisoners are being detained. The US did arrest a handful of Iranians deputed to the compound of Shiite cleric Abdul Aziz al-Hakim and to Irbil, the power base of Kurdistan President Massoud Barzani. These Iranians were there at Iraqi invitation. The US can only interfere here because it has a big force in the country. A small US military force could do nothing whatsoever about Iranian influence in Shiite Iraq, especially in the face of Iraqi Shiite and Kurdish desire for such cooperation. There will be millions of pilgrims coming back and forth, and they all can't be monitored. The major Shiite party, the Supreme Council for Islamic Revolution in Iraq, is tightly linked with Tehran even while being among the main US allies. Small US units trying to take on Iranians in the Shiite south would risk being massacred by thousands of angry Iraqi Shiites...."