Two decades ago, President George W. Bush denounced the Taliban for “aiding and abetting murder” after the terrorist attacks of Sept. 11, 2001.

Now, as President Biden ends the war Mr. Bush started, the United States is locked in an uneasy partnership with that very enemy force, relying on the Taliban to help safeguard American citizens and their Afghan allies as they race to evacuate the country.

It is an improbable battlefield relationship: Diplomats, spies and military officials for the United States are having cooperative discussions with their Taliban counterparts, as the group serves as America’s first line of defense at the airport in Kabul, the Afghan capital, screening passengers for documents and weapons.