IN DECLARING martial law Tuesday, Thailand’s army chief, Gen. Prayuth Chan-Ocha, implicitly asked the Thai people and their friends abroad, especially in the United States, to give him the benefit of the doubt. His move was not an outright military takeover, which would have made it at least the 12th coup d’etat for Thailand since 1932 , but rather a gentler sort of intervention — an attempt to broker peace between the quarreling political factions that have brought violence to the streets and nearly paralyzed the Thai government and economy for the past six months. Certainly the Obama administration was willing to suspend judgment on the general’s intentions, or so it seemed from Washington’s reluctance to brand his declaration a coup, which would trigger mandatory sanctions under U.S. law.

On Thursday, however, Gen. Prayuth forfeited whatever claim to forbearance he might have had. He ended a meeting of political leaders after little more than an hour, pronounced himself in charge of the country, detained officials of the elected government and shut down independent media. The general explained that his action was temporary and intended only to “restore peace,” as military strongmen generally do in such situations. But there is no getting around his disruption of the constitutional order in one of Southeast Asia’s most successful societies, and that’s something the United States and its allies cannot tolerate. It is an ominous development in a region where democracy has put down roots in recent decades but is still at risk in many places.

To his credit, Secretary of State John F. Kerry called the coup by its true name Thursday, adding, in a written statement, that officials are “reviewing our military and other assistance and engagements, consistent with U.S. law.” This is an improvement over the administration’s glaring failure to call out Egypt’s military coup. Mr. Kerry stopped short of actually cutting off aid to Thailand, as U.S. law prescribes. Fair enough: It took nine days for the Bush administration to suspend $24 million in aid after a similar event in 2006; but that precedent suggests a maximum grace period this time, not a minimum.

The United States’ forthright designation of the coup and, if it happens, the suspension of aid are necessary to a democratic resolution, but they are far from sufficient. The Thai army was not acting as an honest broker between factions. It is a politicized force that has long leaned against Thaksin Shinawatra, the populist former prime minister whom the army first ousted in 2006 and whose sister Yingluck Shinawatra was subsequently elected as his de facto substitute. Bangkok’s traditional royalist elite, including the officer corps, does not like Mr. Thaksin or the northern, rural people who support him. But the latter keep winning fair elections.

The United States should instruct its old friends in the military that, unless Thailand’s traditional powers-that-be reconcile themselves to the popularity of the Thaksin movement and the genuine interests of its supporters, this latest coup may yet prove the prelude to a civil war that its authors say they want to prevent.