Donald Trump has all but clinched the Republican presidential nomination, kicking off a US general election campaign between two elderly one-term presidents, each of whom is profoundly unpopular in their own way. Perhaps more unusually for the US, however, it also kicks off an election in which the foreign policy stakes are far higher than usual, and one in which the two presidential rivals offer radically different visions of the US’s role in the world.Political scientists have long known that American voters do not typically vote on foreign policy issues, which pale in comparison with…