I don't think Trump really knows much about anything he's talking about. Regardless, when he speaks he's preaching to the peanut gallery, trying to please his base. That's the one constant with Trump. If indeed this is his vision of foreign policy it is definitely a departure from American policy stretching back for decades. He's basically saying all countries should pull back and fend for themselves. I know his supporters love that idea, and not surprisingly, Putin loves that idea as well. I also found his remarks about 'destroying' North Korea a dumb, immature comment, something you'd hear in a bar after a few rounds. Completely destroying North Korea would also impact hundreds of thousands if not millions of our allies in Japan and South Korea, not to speak of the 30,000+ US servicemen stationed there. Schoolyard bully talk, not fit for a US president. But that's nothing new, nor is it a surprise it was well received by the MAGA redhats.
This is a good run-down on the speech, from right, left and center. I presume you'll agree with the opinions from the right. I agree with the ones from the center and the left. >
https://www.nytimes.com/2017/09/20/us/politics/trump-un-speech-right-and-left.html?mcubz=3