With stunning speed, the Taliban’s lightning onslaught across Afghanistan reached the capital Kabul itself. By late Sunday, President Ashraf Ghani was said to have left the country. Pictures of helicopters lifting off from the US embassy recalled the humiliating fall of Saigon in 1975 — something President Joe Biden dismissed just last month as implausible. This is a tragedy for the people of the country, and a betrayal of the thousands of US and allied troops — and more than 120,000 Afghans — who died in 20 years of war. It is a grave setback, too, for the credibility of the US and of the community of democracies Biden hoped to cement.

It was former president Donald Trump who announced US troops would leave by 2021 provided the Taliban met the terms of a peace accord signed last year. But going ahead with the pullout was Biden’s choice. The domestic political cost is still likely to be low. Polls show Americans are as weary today of the “forever wars” as they were under Trump. Preserving the messy military stalemate into which Afghanistan had settled was a hard political sell. In terms of America’s global standing, however, the miscalculation will haunt the rest of the Biden presidency.

Biden might still have been able to argue for much reduced US forces — which since 2015 had also suffered much reduced casualties — to remain as a backstop to the Afghan military, just as the US retained a long-term presence in Germany and South Korea. Instead, either the White House went ahead with the pullout regardless of intelligence warnings of what would follow, or the speed of the offensive was indeed unforeseen — a startling lack of insight in a country where America has had a ground presence for two decades.