ISIS in Afghanistan claimed responsibility for the attack, which a spokesman for the Taliban condemned. But the Taliban were supposed to provide security outside the airport perimeter, and they failed if they tried at all. Gen. Kenneth McKenzie, head of Central Command that is supervising the evacuation, said Thursday that the U.S. has depended on the Taliban for security screening outside the airport since mid-August.

“We thought this would happen sooner or later,” Gen. McKenzie added. He said the U.S. had no choice other than to interact with Afghans moving through the airport gate to be evacuated, and one of them was probably the suicide bomber who made it past whatever screening the Taliban did.

What a position for the U.S. to be in: Relying on the victorious enemy that has spent years trying to kill Americans to detect jihadists bent on killing Americans.

The Taliban may be enemies of ISIS, as they compete for jihadist supremacy, but they may also believe they benefit from the airport attack. The bomb didn’t kill them. It killed Americans and Afghans attempting to leave the country, and the result will be that fewer Afghans who were allies of the U.S. and NATO will be able to make it out.

Americans have a right to be angry over these deaths. The obligation of a President is to provide adequate force to execute a mission and protect the troops carrying it out. Even after the collapse of the Afghan military, Mr. Biden could have introduced enough force to retake the large Bagram air base, which is further from Kabul and has two runways and a larger security perimeter.

Mr. Biden said Thursday his military advisers told him that Bagram didn’t provide much advantage over the Kabul airfield with its single runway. But even if that’s true, he could have provided more force protection for the airport and an evacuation that wasn’t rushed to meet the Taliban’s timetable.

Mr. Biden has said over the last two weeks that he chose to withdraw from Afghanistan to avoid more casualties. Yet the 13 American deaths—12 Marines and a Navy medic—are more American deaths in Afghanistan than in all of 2020 when thousands of U.S. troops were in the country advising Afghan forces. By one count they are the most U.S. troops killed in a day since 2011.

The risks also aren’t over, as Gen. McKenzie made clear on Thursday. The active threats include truck bombs, RPGs and rockets against aircraft with evacuees. The Journal reports that an Italian C-130 transport plane had to take evasive action when it was fired upon while flying out of Kabul airport.

Thursday’s attack all but guarantees that thousands of Afghan allies and some Americans will be left behind when the evacuation ends. Some European countries have announced they are ending flights by Friday. The U.S. had already warned Americans to stay away from the airport on Wednesday amid security threats. An estimated 1,000 or so Americans are still in the country. Some of them may have to rely on diplomacy with the Taliban to get out once all American troops leave on Aug. 31. In other words, they will essentially be hostages.

On Aug. 20, Mr. Biden stood at the White House and declared: “We’ve made clear to the Taliban that any attack—any attack—on our forces or disruption of our operations at the airport will be met with a swift and forceful response.”

Such a threat might work against the Taliban but it was never going to deter ISIS, whose core ideology is to die for Islam. Mr. Biden is still dependent on the goodwill of the Taliban to complete the U.S. evacuation, so he can’t retaliate against them. He said he’ll find the ISIS plotters and “make you pay,” and he will try.

But the Kabul airport massacre compounds the humiliation of the botched Afghan withdrawal and will further embolden jihadists. Mr. Biden is telling Americans that Afghanistan won’t again become a terror haven, but it already is. The hundreds of jihadists released from prisons with the Taliban victory are already on the attack. More Americans will become targets—and far beyond the borders of Afghanistan.