The killing of the top Qaeda leader offers lessons on 20 years of U.S. involvement in Afghanistan.

The killing of Ayman al-Zawahri in Afghanistan — where planning for the Sept. 11 strikes began more than two decades ago, where the West once seemed poised to remake a fractured nation, and where the terrorist leader could feel comfortable again after the Taliban takeover last summer — speaks volumes about what America accomplished in a 20-year experiment. It also says a lot about where it failed.

On one level, it was a reminder of how little has changed. The Taliban are once again in charge of the country. They were harboring the known leader of Al Qaeda, just as they were 21 years ago. He was comfortably established in a safe house there, so comfortable that his family was nearby, and he had routines to take in the sunshine.

On another level, it was a reminder of how surveillance, drones and remote killing have changed the nature of the hunt for the terrorist group’s leadership. In 2001, America’s drones were largely still unarmed. In the ensuing 21 years they became armed, and the C.I.A. and the U.S. military perfected the art of hunting what they called high-value targets.

To get al-Zawahri took patience — two decades of patience. It validated President Biden’s commitment that, even after withdrawing U.S. troops last year, he would continue counterterrorism operations.

Which takes the story to one more lesson: If the original objective of going into Afghanistan was running these kinds of operations — finding the masterminds of the Sept. 11, 2001, attacks and the generation of terrorists who followed — then maybe it was possible to pursue the mission without trying to remake the country.

But the mission morphed. President George W. Bush celebrated the first inklings of democracy — elections — and the fact that girls could go to school. Military units helped irrigate the fields and built a court system. For a while, America imagined it was building a noisy, nascent democracy. But somehow it never took hold. Drones could not remake the underlying society, or rout the Taliban, who, in many different forms, have always existed. America succeeded at the tactical, but not at the strategic. Bin Laden and al-Zawahri were brought to justice, but just as the British discovered in the 19th century and the Soviets in the 20th, the society proved far harder to alter. Al-Zawahri is gone. The Taliban still rule.