The killing of a prominent Houthi leader in Yemen by a Saudi airstrike this month has prompted threats of retaliation by the Houthis and other pro-Iranian militants in the region, including threats specifically against Crown Prince Muhammad bin Salman (known as MBS). Could that raise new questions about Saudi leadership succession?

Pledging tit-for-tat

Over the weekend, Houthi Supreme Political Council head Salah al Samad was buried in Sanaa. He was killed in a bombing attack a week earlier in the port city of Hodeidah. Samad is the highest-ranking rebel leader killed so far in the more than three-year-old war.

Other senior Houthi figures, including their navy commander, have been killed recently. This has sparked speculation that the Saudi coalition is getting better intelligence from loyalists of former President Ali Abdullah Saleh—who was killed by the Houthis last December, when he defected from being their ally. The United Arab Emirates has welcomed the Saleh survivors. Some reports say a UAE drone killed Samad, based on information from Saleh loyalists.

Last week, the Saudi ambassador in Washington, Prince Khaled bin Salman, tweeted that his brother the crown prince had ordered the attack by “the heroes of the Royal Saudi Air Force.” Prince Khaled himself is a former RSAF pilot. The crown prince is also the Saudi defense minister and the architect of the war in Yemen. The ambassador linked the attack to the Houthi missile barrages against Saudi cities.

Regardless of who actually pulled the trigger, the Houthis and their Iranian allies are promising revenge. Samad’s successor as political chief, Mahdi al Mashat, is close to the Lebanese Hezbollah leadership and has promised that the Saudis and their allies—including the United States—will pay for the attack. The top Houthi leader, Abdul Malik al Houthi, has consistently blamed the United States and Israel for the war, along with the Saudi coalition. Pro-Iran Shiite groups in Iraq have promised revenge against the Saudi royal family. The Iranian media has speculated that members of the family will be targeted for assassination.

Line of succession in question?

Saudi Crown Prince Muhammed bin Salman is the obvious front-runner for retaliation. He is the face of the war and is often mocked by the Iranian media. He has a small army of bodyguards to ensure his safety.

But what if he were gone?

There is no deputy crown prince today—the number three position from which MBS became the heir-apparent. The king shows no inclination to select a deputy crown prince, and there is no requirement to have one. In the absence of someone in that position, King Salman would choose a new crown prince.

There is no obvious candidate. The most experienced and capable Saudi is the man MBS ousted, his cousin Prince Muhammad bin Nayef, who has been out of sight since the king dismissed him last year. He is widely said to be under some form of palace arrest. The return of Muhammed bin Nayef would be awkward, to put it mildly.

King Salman could choose a senior prince with strong standing in the family. His half-brother Prince Muqrin is of course the first Crown Prince Salman dismissed, so that’s unlikely. More possible is a figure like Prince Khaled al Faisal bin Abdelaziz, the 78 year-old governor of Mecca, for example, who often accompanies the king. Or he could choose another one of his own sons, but that would be significantly less popular in the royal family.

In short, the monarchy would face a situation unprecedented in modern times. Since King Abdelaziz al Saud, known as Ibn Saud, created the modern kingdom a century ago, the line of succession has been fairly clear. His sons have now ruled since his death in 1953. MBS is the clear heir-apparent but there is no Plan B.