Russians are mourning after the worst terrorist attack in the country in two decades.  More than 130 people died when gunmen opened fire at a concert hall on Moscow’s outskirts on Friday night.  It’s a major security lapse, but will it cause more Russians to doubt Vladimir Putin’s leadership? We’ll also explain why the Islamic State group that’s claimed responsibility for the attack would want to strike Russia.  Featured:  Rajan Menon, director of the grand strategy program at Defense Priorities