As an obsessive consumer of books, movies, and TV from the Cold War era about living under the threat of nuclear annihilation, one thing that becomes apparent is that the fear of accidental apocalypse was easily as prominent (or, in the view of some scholars, more prominent than) as the fear that some madman bent on destroying the world would gain access to the Big Red Button. It was broadly recognized, particularly once the hydrogen bomb and the ICBM entered the picture in the early 1960s, that a nuclear war was not "winnable" and that even the evil, bloodthirsty Commie understood that well enough that no sane man would initiate one. Ivan knew that Uncle Sam was ready to hit him back hard enough to return the planet to the Stone Age, and both Eastern and Western political systems were robust enough (in their own very different ways) to prevent the elevation of the kind of lunatic who would say "Screw it, let's all die!" into a position of authority.
The biggest fear, then, was that once a massive apparatus of global annihilation was created humans would somehow lose control of it – that like a careless child who finds dad's gun, we would blow the planet up without intending to do so. On the American side, the heavy reliance on automation and seemingly half-crazed SAC Generals combined with the sheer size of the nuclear arsenal strained the ability of the civilian leadership to ensure that nothing happened that was not supposed to happen. For the Soviets, perversely, it was the lack of effective command and control, combined with the doddering character of their political leadership after Kruschchev, that raised the red flags (pun intended). That the world could come to an end because an American computer or a Soviet radio hookup from the 1930s malfunctioned was more plausible to most people than a cartoon villain President or General launching the missiles with a bloodthirsty laugh.
The most popular and in many ways best depiction of this fear in the arts is, of course, Dr. Strangelove:
Muffley: Well I assume then, that the planes will return automatically once they reach their failsafe points.
Turgidson: Well, sir, I'm afraid not. You see the planes were holding at their failsafe points when the go code was issued. Now, once they fly beyond failsafe they do not require a second order to proceed. They will fly until they reach their targets.
Muffley: Then why haven't you radioed the planes countermanding the go code?
Turgidson: Well, I'm afraid we're unable to communicate with any of the aircraft.
Turgidson: As you may recall, sir, one of the provisions of plan R provides that once the go code is received the normal SSB radios in the aircraft are switched into a special coded device, which I believe is designated as CRM114. Now, in order to prevent the enemy from issuing fake or confusing orders, CRM114 is designed not to receive at all, unless the message is preceded by the correct three letter code group prefix.
Muffley: Then do you mean to tell me, General Turgidson, that you will be unable to recall the aircraft?
Turgidson: That's about the size of it. However, we are plowing through every possible three letter combination of the code. But since there are seventeen thousand permutations it's going to take us about two and a half days to transmit them all.
Muffley: How soon did you say the planes would penetrate Russian radar cover?
Turgidson: About eighteen minutes from now, sir.
This is, as we have already been reminded with the Taiwan Phone Call Crisis, the fear we once again have to live with as a nation. If for no other reason than self-interest, we can safely assume that Donald Trump does not actually want to get us all killed. The problem is that he's exactly the kind of person who would blunder into it. Shoot first – from the hip, naturally – and ask questions later. Act with an open disdain for forethought. Be "disruptive" and then let others rush in to clean up the mess you just made. This is, as the well worn saying goes, precisely how accidents happen. I'm not worried that Donald Trump wants to start a war. I'm worried that Donald Trump will start a war, at which point his intent will be irrelevant. That refusal to think about things he says and does that was pointed out throughout the campaign is not a bug. It's a feature. He states explicitly that he wants to be "unpredictable" in foreign policy. It doesn't take much imagination to envision how the infamously irony-deficient Chinese or Russian leadership are going to react to Diplomacy by Coked Up 3 AM Twitter Zinger.
"He wouldn't do that" is only reassuring inasmuch as the "he" in question is conscious of the potential consequences of his words and actions. Your dog doesn't want to knock over the glass perched on the edge of the kitchen table, but dammit if he doesn't do it every time he gets excited.