Iraq has seen warfare numerous times before now. British troops entered in WW1, in WW2, they occupied Baghdad and trained many of their troops. Some of these were pro-German and a key battle was fought at Fallujah in 1941 against Iraqi troops. It is the same place where American troops fought one of the bloodiest battles in 2003. Of course, there was the eight year Iran-Iraq war in the 1980s.
Winning in Iraq is now pointless.
We are at the same critical junction that we were at in the Vietnam War in 1970. Despite winning the battles, we cannot seem to win the "hearts and minds" of the Iraqis nor subdue the violence between groups and terrorists. This brings me to the Russian Civil War from 1918-20, when the Allied nations (UK, US, France) thought they could kill the birth of Communism. After a year, the US and France went home, only the British remained until 1920, supporting the White cause in a situation that was corrupt and winless.
History repeats itself over and over.