Duck and cover.

Mutually assured destruction.

CONELRAD symbols for the emergency stations (640 and 1240) on the car radio.

Unwelcome back to these and other memories of my boomer boyhood, those black-and-white days of mushroom clouds on TV and menacing A-bomb and H-bomb figures in Herblock's newspaper cartoons.

President Trump promises North Korean belligerence will be met with "fire and fury like the world has never seen."

But those of us who grew up in the 1950s and '60s have already seen it: in nightmares fed by the frightening photos in Life magazine, the sonorous voice of Rod Serling, and the fact that our neighbors — in my case, a guy named Charlie who lived up the road — were building fallout shelters.

The enemy then was Nikita Khrushchev. We learned to spell his name at my elementary school, where, one gray morning, our strangely solemn principal gathered us together and told us what to do, just in case.

I, however, already knew what to do: I had read "You Can Survive the Bomb,"  a chipper do-it-yourself guide in the Catholic Digest that advised families like ours to stock up on canned goods and fill jugs with water. These were tasks I  instructed my parents to do, and when they actually did them, I was more frightened than reassured.

If the alarm sounded, I told them, all eight of us were to go to the basement — "down cellar," as people in my part of New England used to say — and there, we would live on canned green beans and carefully measured sips of water until the fallout had all fallen down and we could go back to school.

My earnest faith in my survival plan for the Riordans of 39 Burnham St. faded, however, after I read the Reader's Digest version of Hiroshima by John Hersey and borrowed On the Beach by Nevil Shute from the North Adams Public Library. The notion of holing up down cellar until the smoke cleared seemed far less feasible, given  the reality of what nuclear warfare and its aftermath could mean.

Although most of my juvenile obsessions eventually gave way to more grown-up obsessions, in the early 1980s, I was riveted by cautionary tales like The Day After and, especially, Testament, with unforgettable Jane Alexander's character calmly shepherding her dwindling brood until the end.

Those were the days: "No Nukes" was on the sound track, and Ronald Reagan was in the White House. Though his utter silence during the first five years of the AIDS epidemic was unforgivable, he did in fact preside over the beginning of the end of the cold war.

The Iron Curtain that so grimly shadowed the screen of our Motorola was lifted, for real, in no small part due to Reagan's leadership.

As we all know, too many other nightmare scenarios — 9/11, mass murders by gun-crazed domestic terrorists — have evicted nuclear warfare from the front pages.

Now, the possibility of it is back, invading my reveries early this morning in Haddon Heights Park (my little Arcadia), bellowing from the talking heads on Morning Joe, and from the headlines of the newspaper waiting on my doorstep as I got back from my walk.

Now we have a president for whom calm deliberation and judicious word selection appear to be foreign concepts, a president who unleashes ill-advised proclamations on Twitter, who imagines his job as a mash-up of reality-show star and bully-in-chief.

I do take some comfort in the fact that though he's older than I, Donald Trump also grew up in the era when Americans wondered whether they had room for a shelter in their backyards, or whether death would fall from the sky as their families slept.

Memories of the anxiety that hovered like background radiation during those years are with me again this morning. So I hope that as he puts his finger to the keyboard to send his next tweet, the president — our president — does not forget.