On the two-decade anniversary of the U.S. invasion, Iraq is weakly governed, leaving it prone to instability and meddling by neighbors—especially Iran.

America is one country and two nations. They will have to coexist; whether they can work together remains to be seen.

Alone among US President George W. Bush’s senior advisers, Colin Powell did not push for the costly, ill-advised Iraq War, and it would be misreading history to hold him responsible for it. He remain…