Sure, folks can look back at American history and find bits of Trump-ness in the past, but we tend to view those times as big mistakes, such as internment of the Japanese. FDR gave into fear despite his admonition about fearing fear itself. Yes, the US used to have a mercantilist trade policy, but that is not how we got from being a minor inconvenience to a superpower. The international order the US created in the aftermath of World War II was surely not altruistic, but it was largely aimed at producing a better world than the one that preceded it.
Free trade? Trade has never been completely free, but by reducing the barriers to trade, the US through bilateral deals and multilateral institutions helped the world rebound from the war and the Great Depression. This created markets for American goods and later for American services. It meant, ultimately, that the countries elsewhere would develop some comparative advantages, which led to declines in key sectors of the American economy. Yes, it hurt, but we are far better off with economies that can buy the rest of our stuff, with products made elsewhere that are much less expensive than they would be protected market (clothes, ipads, cars, etc). The decline in poverty around the world is in part due to the American fostering of trade. Don't care about poverty elsewhere? Well, the best way to prevent immigration, if one is intolerant, is to support economic growth elsewhere.
One of the ironies of Trump's ascension in the Republican Party is that he wants to impose tariffs on imports. Tariff is a fancy word for TAX! That Americans would have to pay more for the stuff that they like because there would be taxes on imports. Where is the Republican objection to taxes now?
Security? Yes, the US has fought wars since World War II, but entirely at our choosing and with declining costs. Europe has been stable since 1945, first because the US deterred the Soviet Union and since because the US supported institutions such as NATO to continue to keep the peace and foster democratization. Yes, it costs real money to keep Europe and Japan and South Korea secure, but our allies do pay AND we are not doing it out of altruism. The American economy depends on freedom of the seas and stability in Europe and Asia (sorry, Africa).
Instead, Trump wants to undermine the security architecture and solve problems by having heaps of meetings with Putin and other autocrats he admires. Selling out the allies to a Russian authoritarian leader is, yes, un-American. Running alliances like protection rackets, "hey, it would be really sad if something happened to you, Estonia, if you didn't pay up ....," is not the American way. Burden-sharing is an issue, certainly, but this is not the way to get the allies to pay the bills. While we can be skeptical about credibility and resolve as the keys to American foreign policy, alliances do matter, and the credibility of the US commitment to its NATO partners is important. They literally sacrificed lives for us, as they were mostly out of the fight on 9/11, but joined the US in Afghanistan anyway.
How Trump proposes to lead the US in the world is exactly what the US does not stand for. Exploitation of allies? Coercion of democracies (the US tries to coerce authoritarian regimes)? Striking up mercantilist deals (so much for NAFTA)? These are not the American way.
While I remain confident that Trump will lose and probably in a landslide, the damage he is wreaking within the US via inciting violence and empowering white supremacists and outside via undermining America's standing in the world will endure past November. And that does, indeed, make me sad.