STUTTGART, Germany — If the U.S. has a “Plan B” in the event Iraqi Prime Minister Haider al-Abadi is toppled from power, Defense Secretary Ash Carter was not sharing it Monday.

The Iraqi capital was shaken Saturday when protesters loyal to the firebrand Muslim cleric Muqtada al-Sadr overran barricades of Baghdad’s fortified Green Zone, stormed the parliament and staged a sit-in before leaving the next day.