Barack Obama believed his legacy as president would be that he ended the Iraq war. It looks increasingly that his legacy could be that he lost it.

By their admission, President Obama and Vice President Joe Biden inherited a war that had been won. In 2011 Mr. Obama said America was "leaving behind a sovereign, stable and self-reliant Iraq," and Mr. Biden proclaimed Iraq "one of the great achievements of this administration."

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Mr. Obama then committed a massive error in judgment by withdrawing all U.S. troops. That allowed the rise of the Islamic State of Iraq and al-Sham, or ISIS, the world's most formidable, merciless and dangerous terrorist army. ISIS is now establishing an Islamist caliphate that stretches from Aleppo in Syria to Fallujah and Mosul in Iraq and beyond.

The president was warned about the ISIS threat for months, yet did essentially nothing. As in so many other important moments when decisive action was required, his actions were haphazard and reactive, his leadership detached, his will almost nonexistent.

To continue reading Karl Rove's column in the Wall Street Journal, click here.