Obama’s longer-term foreign policy challenges

With 2½ years left in his presidency, Barack Obama finds a series of daunting global crises filling his Oval Office in-box. Countering Vladimir Putin’s aggression in Ukraine and Iran’s drive for nuclear weapons are top priorities. He must also do more to help the millions of refugees from Syria’s bloody civil war. And he and Secretary of State John Kerry may choose to circle back to the faltering Israeli-Palestinian talks. But his foreign-policy legacy will also depend on how successfully he tackles the longer-term global challenges on the horizon. Here are four that will test Obama (and his successors) in the years ahead.