The factor that was most responsible for the fall of Afghanistan was President Trump’s decision to announce the withdrawal of US forces from Afghanistan, even without consulting Afghan leaders or Western allies. His decision to hold talks with the representatives of the Taliban in Doha behind the back of the Afghan government emboldened the Taliban and undermined the morale of Afghan leaders and armed forces. It seems in this case, America first became America only. In his various interviews with the media, Tom Tugendhat, chair of the Foreign Affairs Committee of the British Parliament, said that negotiating with the Taliban behind the back of the Afghan government was as if US officials had negotiated with IRA terrorists at the height of the Troubles behind the back of US officials about the fate of Ireland. He complained: “we abandoned the Afghan people”.

Source: Top Nine Policy Recommendations for U.S. in wake of Afghanistan Debacle