"The fundamental principle that a Government's duty is entirely the interests of its own people, does not. We have gotten into an Alice In Wonderland pursuit of changing other peoples, which is as immoral as it is insane."
Ok, quick yes/no question - a regime patterned after Nazi Germany is freely elected in Mexico. They immediately assure us that they have no intentions of doing any harm to the US, in fact they take real steps to help stop the flood of illegal immigrants crossing the border.
They also immediately begin opening up the concentration camps and firing up the ovens for all the usual political prisoners - Jews, homosexuals, etc.
Does the US take military action, yes or no?
YES or NO?!?!?!
Traditional American foreign policy was not "isolationist." It was based upon understanding what is one's business and what is not. It was based on offering the rest of the world, trade, friendship and respect. In turn, we demanded respect back. If we did not get it, we responded appropriately--at times, to paraphrase Jefferson, it was necessary "to punish the first insult," but we never imagined it was our business to meddle in other people's cultures. We were willing to lead by example, if someone wanted to observe our example, but that ultimately was their business. Ours was--and still is--the fundamental ongoing interests of our own people, down through the generations.
The Dean Rusk foreign policy (1961-1969) advocated promoting major change in other lands. The present Administration has revived that. You apparently think that well and good; but it is difficult to see how it differs from the Communist and Nazi policies of the last Century.
Your hypothetical situation, with respect to Mexico, is a pipe dream, which shows profound ignorance of the nuances of both German & Mexican societies, as well as the actual dynamics that brought and kept the Socialists in power in Germany (1933 to 1945), as well as the origins of the demonization of Jews by German Marxists. (By the way, FDR's "New Deal" was far closer in many respects to the Nazi approach in Germany, than anything in Mexico. Policies must be based on real situations, not pipe dreams.)
If you think we are supposed to be the world's policeman, I would suggest you find some rational argument for our being such. Your wish to have intervention against all evil is not a rational argument.
For a more detailed statement on foreign policy:
William Flax