The former head of NATO has said the UK military cannot return to Afghanistan after the country was retaken by the Taliban on Sunday. Lord Robertson, who served as general secretary of the military alliance between 1999 and 2004—and issued a statement invoking article five of the treaty that provides for collective defence after the attack on the World Trade Centre on 9/11—said on Monday that the UK should focus on increasing its defences against terrorism at home. The Taliban marched into Kabul on Sunday, taking the presidential palace with little resistance, while President Ashraf Ghani was forced to flee. Lord Robertson said on “Good Morning Scotland” on Monday that the speed of the takeover showed a “failure of intelligence” from allied forces. He said he was “sad and sickened” by the scenes from the country, where coalition forces had been stationed for 20 years. “I find it ironic at …