Touting his four years as secretary of state, John Kerry on Thursday attempted to explain one of the most contentious moments in American diplomacy during his tenure: President Barack Obama’s failure to enforce his “red line” warning to Syria about using chemical weapons. In a news conference meant to promote his achievements, Kerry said that Obama didn’t backtrack in 2013 on his ultimatum to Syrian President Bashar Assad not to attack rebels or civilians with his chemical weapons arsenal.