The US has spent billions of dollars on Afghanistan over the past two decades, but what has come out of it? As US and allied troops withdrew from Afghanistan, the Afghan government propped by the US seemed to have vanished into thin air. Why has the situation in Afghanistan rapidly devolved?  What drives US foreign policy? CGTN Host Tian Wei had a candid and insightful conversation with Jefferey Sachs, professor and director of the Center for Sustainable Development at Columbia University.