Who the Americans put in the White House makes a big difference to the world because US presidents have considerable power to shape foreign policy.
Think Lyndon Johnson and Vietnam; Richard Nixon and the opening to China; George Bushes Senior and Junior and their Iraq wars.
This election offers voters a real choice. Despite Donald Trump’s sometimes incoherent and seemingly improvisational approach to foreign policy, the two candidates do offer quite different visions. Hillary Clinton firmly believes the US role is to uphold a global security order from which it also benefits, the Pax Americana at the core of traditional US military and diplomatic thinking.
Donald Trump’s America First approach is more transactional. He frames alliances in business terms, vowing to get better value from them or pull back from historic commitments he says the US can no longer afford.
This is how their differences might play out in key international issues.
In no other area has Donald Trump departed more radically from decades of US foreign policy than his approach to traditional relationships. He has castigated Nato as outdated and obsolete and characterised its members as ungrateful allies who benefit from US largesse.
He says America can no longer afford to protect countries in Europe – and in Asia – without adequate compensation, suggesting he would withdraw American forces unless they pay up.
He’s also said Nato members like the Baltic states couldn’t count on the US to come to their military aid if attacked by Russia, unless they’d fulfilled their obligations.
Mr Trump is voicing longstanding criticisms. President Obama has also expressed frustration that most Nato members don’t meet their goal of spending at least 2% of GDP on defence.
But Mr Obama stands firmly by the military alliance. As does Mrs Clinton, who proclaims Nato one of the best investments America has ever made.