Russia has charged the four men it says attacked a popular Moscow concert hall with an act of terrorism, which ISIS has claimed responsibility for. 

Russia's Vladimir Putin has acknowledged the attack - Russia's deadliest in two decades -was conducted by radical Islamists, but also tried to link it Ukraine's attacks on Russia without evidence. 

Guest Anatol Lieven is the Eurasia director at the Quincy Institute for Responsible Statecraft and a former British correspondent in Afghanistan, Pakistan and Russia