The idea of the frontier has been central to American identity. Its image of endless promise fostered our belief in the US as exceptional. Today America has a new symbol: the border wall. As GREG GRANDIN sees it, America’s constant expansion helped deflect domestic political and economic conflicts outward. But the 2008 financial crash and unwinnable wars in the Middle East turned Americans inward, leading to the rise of reactionary populism, racist nationalism, extreme anger, polarization - and Trump.