It looks like Iran. A lengthy article in the New York Times shows how Iran has become dominant in Iraq. That's the legacy of our ill-considered war to overthrow Saddam Hussein.
The Times notes that Iran not only decides who has power in the Iraqi government but also is using its position to provide military support to pro-Iranian forces in Syria and Lebanon.
It is not surprising that a Shiite-majority Iraq would have cordial relations with Shiite Iran, but Tehran's dominance makes it much harder to build a unified country where Sunnis and Kurds feel welcome and safe.
America's defeat has many causes, but the most valuable lesson we can learn is, Be Careful about going to war.