The balance of peace in the nuclear age is a lot more fragile than people realize.
The issue here is not whether you know Trump will try a nuclear wag the dog, or whether Kim Jong Un is actually considering nuking Guam. The point here is instability.
Wars are very often not planned events, or, at best, they are half assed plans. Nobody wanted the First World War; it was a cascade of events and bad decisions that led to Europe trying to slit its own throat. The Falklands War was a case of two countries failing to understand the actual policy of the other. The First Gulf War happened because Saddam Hussein was inadvertently given the impression the USA would not seek to liberate Kuwait. I could go on and on.
All levels of uncertainly when it comes to nuclear war are unacceptable. The consequences of a mistake aren't just bad, they're catastrophic to our species. That is why, during the Cold War, the lines were quite clearly drawn between NATO and the Soviets/Warsaw Pact; this is our side and this is your side, and if you cross the line, God help us all. The Soviets were not going to invade West Germany when they knew the result would be nuclear war, and vice versa. I'm not discounting the terrible human toll of the proxy wars fought outside Europe, but the peace was kept there, and the missile kept in the silos, by virtue of the fact that it was very clear to both sides what the other side's policy was.
What Trump has done, stupidly, is make American policy unclear. Is he going to attack North Korea for threatening? He said he would, but now he's not. Is he willing to go to the mattresses if Russia attacks a NATO member? Up until January 20, 2017, it was clear that the USSR/Russia attacking a NATO country was suicide; now, maybe not so much. I'm not saying I know Putin wants to invade Poland, but the thing is that in 2015 Vladimir Putin, who is not dumb, knew doing that would result in a devastating retaliation and the likely annihilation of his country. Now, well, he's not so sure, is he? And if Trump's not possessed of a great understanding of the value of making the tripwires clear, it's very likely he doesn't get what OTHER countries' tripwires are, and would be surprised that China might take offense to an attack on North Korea. Of course I am sure the adults in the room are trying to get that across to him, but he doesn't listen to things he doesn't like to hear.
Trump, like many idiots, believes he's playing a game of deals and winning and losing where being unpredictable is a virtue, and pulling off an unexpected move leads to winning. War and peace are not games, and history bears out that unpredictability leads to tragedy.
The difference now is that in 1914, when some loser from the sticks committed a murder that started a fire no one could put out, about sixteen million people died. If a mistake is made today, billions of people could die.