One of the first things to consider is that nobody really wants to go to war. Stop worrying about Tom Friedman and Bill Kristol. They have no control over the military apparatus, and even Trump will have to make a case for war and accept a scenario acceptable to the likes of Mattis and McMaster, and they are far from suicidal.
But it's actually much more than that. One of the common talking points we lefties always point out is the wildly disproportionate military spending of the United States. We always rush to point out that the US spends more on its military than the next ten nations combined. It's not even close, and I think we all agree this is an unequivocal waste of money. But it is real, and if you start talking about war, then the calculation changes. How many nations would really want to face that? The simple fact is that the US can rain hell and destruction on any nation that wants to step up to the plate. It's true we've struggled with counter-insurgency and trans-national terrorist groups, but give us a nation's infrastructure and there's simply no better force on earth when it comes to wrecking stuff.
No nation - not Russia, not China, not even the Generals in North Korea - want to face the full wrath of a US in full 'blow 'em to hell' mode. After the F-22s knock out your fighters and the Wild Weasels and cruise missiles wreck your anti-air assets, you get US jets overhead 24x7 just hunting targets, wrecking infrastructure, killing leadership. Anything you put on or under the ocean will be wreckage on the bottom in less than a week, and anything you try to protect will be detected and destroyed because war is nothing less than a gigantic tantrum and the biggest guns always win.
It's also very important to understand why there have been precisely ZERO major power conflicts since 1945. The answer is those pesky nukes. Nobody - not the US, not Russia, not China, not Europe/NATO - wants to risk anything that might lead to out-of-control escalation. The risk of nuclear conflict isn't driven by starting wars - it's driven by ending them. In a major power conflict today, any side that came to recognize that they were losing would be likely to try to avoid such an existential outcome by using tactical nuclear weapons on the battlefield and trying desperately to limit the extent of the exchange. But nobody wants to find out if that might be possible without killing a billion people and turning dozens of major metropolitan areas into radioactive wastelands.
(I often gaze up at the stars at night and wonder how many planets are out there that had intelligent, even brilliant populations and now are smoldering ash because their clever denizens figured out how to split the atom.)
In the end, it's most likely that even a blustering ignorant buffoon like Trump can't change the overall global geopolitical calculation. Make no mistake, he'll do some stupid things and people will die as a result, but no other national leader is going to let Trump goad him into national suicide.