Trump ran on an "America First" platform. I don't know what he means by that. We'll find out, based on what he actually does, or attempts to do.
His slogan (which doesn't originate with him) raises the question of whether it's good or bad for a US president to have an America First policy.
i) Obama is a self-hating American. He used to go around the globe, apologizing for American history and foreign policy. He presumed to speak on behalf of Americans generally.
One problem with his groveling behavior is that he typically issued these apologies to countries with hideous human rights records.
ii) America First doesn't imply that America is better that every other country. Rather, it's a statement of priorities. The job of an American president is to act in the best interests of his constituents. He's an elected representative of the American people. He's supposed to work for their benefit.
iii) And that's a viewpoint which other heads-of-state are supposed to assume in reference to their own countries. The Scottish Prime Minister should have Scotland First agenda. And so on and so forth.
iv) Nationalism doesn't imply that you think your country is the greatest country on earth. I don't think any one country has all the best things.
However, some countries are definitely better than others. Some countries are horrible places to live. There are superior and inferior cultures.
v) Putting national self-interest first doesn't imply an imperialistic outlook, where you conquer and subjugate other countries for their natural resources, to enrich your own country. And what's good for your own country may be good for other countries as well.
vi) America First doesn't imply isolationism. For instance, countries often enter into formal or informal military alliances. Take information sharing between the intelligence agencies of two countries. That can be mutually beneficial to the national security of each country.