Here's the way I figure it...and credit where credit is due, I got this viewpoint from a chapter in Henry Kissinger's classic book Diplomacy - I highly recommend reading it if you haven't already.
Basically, there are two schools of thought where it comes to American Foreign Policy... Wilsonian Idealism and (Teddy) Rooseveltian Realism. Basically, Wilsonian Idealism involves the fostering of regional alliances and intervening on behalf of weaker allies when they are threatened by stronger neighbors. It was pretty much the dominant ideology of the Cold War. Rooseveltian Realism divides the world up into spheres of influence dominated by the great powers... it involves taking a hard-eyed look at the world and only intervening when our direct national interests are threatened. A Rooseveltian Realist probably would have intervened in Korea because of the threat to Japan (being, at the time, within our sphere of influence) but would have taken a dim view of intervening in Vietnam (viewing it as being more within China's sphere of influence). If the Soviet Union wanted to contest Vietnam falling into China's hegemony, it could have done so and opened up the split between the two nations and the US could have stayed out of it. Hence, Nixon's "triangulation" strategy.
Of course, being the real world, I don't think all that many of our Presidents have been "purists" where it comes to these two strands... their policies have tended to fall somewhere between the two extremes. Kennedy and Johnson's actions in Vietnam were probably more Wilsonian while Nixon's were probably more Rooseveltian, but you get the basic idea.
Where it comes to Trump, I think he's probably the most Rooseveltian we've had since Nixon - even more so than Nixon. He has little use for NATO because that's a textbook Wilsonian organization. The EU is a Great Power, and so should act accordingly by strengthening it's military and establishing it's own sphere of influence. The same with Japan. Not seeing Syria as being within the US sphere of influence, he is more than willing to leave it to the Russians to deal with.... but Russia herself is going to have problems there. Syria is a proxy fight between Saudi Arabia and Iran... and Turkey, for that matter. One of those three will end up the dominant power in the region and probably the next Great Power. Not Trump's concern, though.
You want to know how a Rooseveltian Realist as PM of the UK would have dealt with Hitler? By drawing a line at Western Europe.... stay out of Flanders and Scandanavia, and we'll be fine. Let the Soviets deal with them. Eventually Germany's sphere of influence would have come into conflict with the Soviets and they would have fought it out. Maybe the winner would have been weakened and easier to deal with... or maybe the winner would have emerged as the world's dominant superpower? Who knows? It would have been a very different world, though.