After 9/11, we as a nation were forced to recognize that potential terrorist threats could in no way be ignored. Aside from Osama bin Laden, Saddam Hussein was the biggest threat to US security, and thanks to our efforts in Iraq, he was eliminated.
We went into Iraq looking for weapons of mass destruction, which we did not find. Although the threat turned out to be empty, it is better to be safe than sorry. Had we not taken immediate action and had there been WMDs, we would be living in a completely different world.
Pulling out of Iraq now would be a major threat to our national security. Like Germany after WWI, Iraq would be left without the proper resources to rebuild their country. History could repeat itself if we make this mistake again. We do not want to create an environment whereby Iraq could generate the same kind of fear that was generated by Germany in the last century.
Regardless of whether or not we agree with the reasons for entering this war, we cannot remove our troops until Iraq is fully established as a new democratic nation. We do not want our children to grow up as the targets of future Iraqi hostility.