Britain’s defense secretary, Ben Wallace, and some British generals had been vocal in their criticism of the American policy, dating back to Mr. Trump’s outreach to the Taliban and his initial announcement of an American withdrawal in February 2020.Mr. Wallace has said Britain sounded out other NATO members about the possibility of organizing a stabilization force in Afghanistan after the United States left. That idea went nowhere, and even if it had, security experts said a NATO force without American participation would never have been sufficient to hold off the Taliban insurgency, given the massive air power required.