In a well-written essay in The Atlantic David Kennedy of Stanford explains:

“From its birth, the United States thus infused its diplomacy with a revolutionary ideology that looked to the creation of a novus ordo seclorum in the international sphere as well as the domestic. That ideology constitutes the core of the idealist tradition in foreign policy. It is value-driven, morally infused, and devoted to concepts of international law and lasting peace. From its inception, idealism melded with an inherited sense of America’s providential mission to redeem the world. It also fitted easily with America’s emerging culture of mass democracy, and adumbrated some conspicuous facts about all popularly elected governments thereafter: that democratically accountable leaders must justify their foreign policies on grounds that the mass of citizens will accept—and that self-interest alone often appears inadequate to that end. In democracies, history attests, a measure of idealism may be necessary if a state is to sustain a coherent foreign policy.”