MOHAMMED BIN SALMAN, the Saudi crown prince known for his bold and sometimes reckless steps to transform his country and its place in the Middle East, has staged his biggest coup yet. On Saturday, 11 princes and three dozen other senior officials and big businessmen were arrested on his orders, including the commander of the national guard, the economy minister and Saudi Arabia's leading international financier. Though they are reportedly being held in luxury hotels, Saudi media are describing them as "traitors," an ominous sign of what may be in store.

As if that were not enough for one weekend, the 32-year-old prince flexed Saudi muscles across the region. Lebanese Prime Minister Saad Hariri, a client, announced his resignation on a Saudi television channel, triggering a crisis with Iranian-backed Hezbollah, which was part of his coalition. The Saudis then declared a blockade of Yemeni airspace and borders after blaming the Tehran-allied Houthi movement for a missile attack on Riyadh's airport. On Monday, Prince Salman abruptly summoned Palestinian President Mahmoud Abbas, who has been negotiating a political pact with the Hamas movement.

The sweeping initiatives appeared aimed at consolidating the power of an emerging ruler who is poised to succeed his 81-year-old father, King Salman, and to double down on Saudi Arabia's challenge to Shiite Iran. The young prince appears to have the full support of President Trump and senior advisers such as Jared Kushner, who visited Riyadh and conferred at length with the crown prince only days before the crackdown. If so, it's a risky bet.

His Western supporters imagine that Prince Salman is concentrating power so as to lead his deeply conservative, oil-producing country on a forced march to modernity. Prince Salman has announced plans to welcome foreign investment to the country and to sell shares in the state oil company; Saudi women were recently told they would soon be allowed to drive. A wave of arrests in September swept up many conservative clerics.

Yet Prince Salman's resort to heavy-handed tactics and ill-judged adventures could easily undermine hopes for progressive reform and destabilize the kingdom. He is arresting not just hidebound Islamic clerics but liberal journalists and human rights activists who ought to be his natural allies. His jailing of globally prominent businessmen such as Prince Alwaleed bin Talal, a major investor in companies such as Apple, Twitter and Lyft, could spook the foreign investors he says he wants to attract.

Most seriously, the war the young prince launched in Yemen in 2015 has become a quagmire and fuels what is probably the worst humanitarian crisis in the world. The Saudi-led boycott of Qatar has introduced a rift among Sunni-ruled Persian Gulf states, to Iran's advantage, and it too has produced a stalemate. Mr. Hariri's resignation will likely have the effect of strengthening rather than weakening Hezbollah's hold over Lebanon.

To the extent it backs Prince Salman's gambles, the Trump administration is needlessly risking U.S. equities in the Middle East. Rather than indulge the young ruler, it ought to be pushing to curb his excesses, both at home and abroad.