After a long rise from the early Colonial period, the expansionist phase of America's geopolitical influence peaked sometime between WWII and the end of the Cold War, and no later than the 1990s the Establishment began grasping at straws to create the illusion that we were still a British Empire for the new millennium. As in so many other domains, Trump's goal in foreign policy is to end the wasteful foolishness and get real. (Read a transcript of his recent speech, or watch the video.)
This shift will be one of the main differences between today and the last time a populist Republican led a re-alignment out of a period of laissez-faire economics and open borders -- the election of 1896, won by William McKinley, Trump's closest predecessor.
Back then, America was still expanding: just one year into his Presidency, McKinley presided over the Spanish-American War, through which America gained Puerto Rico, Guam, and the Philippines, as well as a protectorate over Cuba. We intervened in Latin American politics for the better part of the 20th century. In 1896, Utah had just attained statehood, and there were still five states out West left to join (Oklahoma, New Mexico, Arizona, Alaska, and Hawaii).
Today, our Union isn't going to get any bigger, and with the prospect of a Puerto Rico bailout, if anything the American people will be open to shrinking it. And we see what we get by trying to maintain a sphere of influence over the entire globe.
"America First" is a great way to transition away from imperial over-reach without sounding or feeling defeated. It's not that the rest of the world beat us, rather we beat ourselves through arrogance and wastefulness. Shrinking our sphere of influence down to the 50 states is no more defeatist than the Roman Empire letting go of the eastern Mediterranean and North Africa. It's not going to be the age of the Five Good Emperors forever. And it's not going to be the age of Teddy and Franklin Delano Roosevelt forever either.
One of the main dangers facing a shrinking empire is getting over-run by migrants -- the Germanic tribes during the decline of the Romans, South Asians being invited into the post-colonial UK, and now the Muslim migration over Western Europe. Peter Turchin identifies the glue holding together an empire as solidarity among the citizenry, or the potential for collective action. When that glue starts to come undone, it will naturally show up in the form of borders not being defended as fervently as in the climate of "we're all in this together".
Thankfully, the Trump movement is working to solve this major problem of post-imperial political life, most of them not knowing anything about the fall of the Roman Empire, and therefore not deliberately "applying the lessons of history". But whether consciously or intuitively, we're largely going to avoid getting over-run by foreign hordes as we shrink back our sphere of influence, once we Build That Wall, deport the illegals and anchor babies and their families, end birthright citizenship, and dial down immigration.
To close on a different topic, it was reassuring to hear Trump use the phrase "Western" values, institutions, and civilization, rather than the phony construction "Judeo-Christian" values etc. that the guru of the Cruz Cult would have used. It's a shift away from the Jewish-influenced neocon agenda of nation-building and cultural imperialism, and setting a more pragmatic and non-interventionist goal of staying true to our (non-Jewish) cultural roots and setting a good example. Whoever wants to emulate us, can try, and whoever does not, will not. Big deal.
When Trump does mention Christians, he's referring to an ethno-cultural group under siege in the Middle East. He doesn't talk about the entire religion of Christianity, let alone as something that ought to be spread around the world in a latter-day version of the White Man's Burden. Leave cultural change to the missionaries, not the politicians. That point generalizes to all aspects of the culture war that has yielded zero results for the would-be conservative theocrats.