The U.S. effort in Iraq is not over. Some 50,000 U.S. troops, together with a robust diplomatic presence, continue to train and assist Iraq's security forces and support its democratic progress. The American people, our coalition allies and especially the Iraqi people have paid an enormous price. It is important to remember why.

For over two decades, the regime of Saddam Hussein had threatened the national security of the United States, its key allies and the stability of the Middle...