America’s core interest remains a stable, united and democratic Iraq. But American regional interests are broader. At stake now is the century-old political structure of the entire region, with huge consequences for our friends and allies there.

If the terrorists continue south and take the capital, Baghdad, or threaten the Shiite holy cities of Karbala and Najaf, a full-scale civil war is likely. Grand Ayatollah Ali al-Sistani Friday issued the first call for “jihad” by the Shiite religious leadership in almost 100 years. Radical Shiite leader Muqtada al-Sadr has reactivated his ” Mahdi ” army and other Shiite leaders have recalled two battalions from Syria to fight in Iraq. A serious threat to the holy cities would almost certainly provoke intervention by Iranian Revolutionary Guards on the side of the Shiites. Kurdish leaders, who have the best-organized military force in Iraq, have taken advantage of the current chaos to wrest control of the long-coveted city of Kirkuk from the central government, and would be tempted to declare Kurdistan’s independence.

Those Americans who have pressed in the past for dividing Iraq should be careful: They might get what they wished for. The price would be very high: a regional war on top of an Iraqi civil war. American action now would be considerably less difficult than later.