A Congressional talking head on CNN or MSNBC this morning talked about a nuclear-armed North Korea as an existential threat to the United States. He needed to specify what he meant by "the United States."
If he meant the American Republic, he definitely had a point: one enemy bomb going off in an American city — hell, one "friendly-fire" nuclear explosion — and many Americans and most of the US government would totally panic, and we'd be under some variety of martial law for the foreseeable future.
So, yeah, a North Korean nuclear capability and their willingness to use it against the US and what is in some ways admirable — an American horror of very large numbers of dead Americans: that combination is a threat to the existence of the American Republic.
North Korean military assets taken all together, including a fair number of nuclear warheads, is not, however, a threat to the United States in the sense of the American state or the American nation — not even if you define the American nation as only real Americans as in White, conservative, Christians (preferably Protestants in the Knox/Calvin/Puritan/Fundamentalist tradition).
During World War II, the Germans under Hitler and Russians under Stalin did a fair job destroying states — Poland for a key example — but that was done as conscious policy and with a few lawyers and many serial killers on the ground, not by bombing cities. When the British and Americans air forces showed what aerial bombing can do and wiped cities off the Earth, that still in itself did not destroy the German state nor the Japanese. And for nations, World War II and its run-up was the time of large purges, massive manufactured starvation, and attempted genocide, but no nation — i.e., a cultural/ethnic people — was destroyed in spite of very ... let's say energetic attempts to do so: not the Jews, not the Roma, nor the Poles.
In a course I finally named just "Massacres," my students reported the disturbing fact that in spite of the casualties of World War II — Matthew White counts some 66 million dead — the human population rose during the period, including in every theater of war except Germany's Eastern Front, what Timothy Snyder calls the Bloodlands. The human population of the United States is over 325 million, and even with the most bigoted, racist, exclusionary definition of "the American nation," we have enough people to survive a million casualties in a limited atomic attack and its deadly aftermath (disease, hunger, survivor violence).
General "Buck" Turgidson in DOCTOR STRANGELOVE (1964) is a sociopath in suggesting a massive, first-strike attack on the USSR, but he has a point on American casualties : "Mr. President, I'm not saying we wouldn't get our hair mussed. But I do say no more than ten to twenty million killed, tops. Uh, depending on the breaks."
What is an existential threat to the United States as Republic, State, or nation; what is an existential threat to human civilization and perhaps the human species is full-scale thermonuclear war. And with full-scale (thermo)nuclear war we're not talking directly about North Korea but first and definitely foremost about the arsenals of Russia and the USA, plus France, China, and the United Kingdom.
To remove the existential threat to the United States et al. we need reductions down to the minimum for deterrence by the major nuclear powers.
Having said that, however, I'll add that I live next door to a deep-water port on the US Pacific coast, and I get to walk past very large container ships and don't have to wait for the North Koreans to develop full ICBM capacity to be concerned about delivery into my neighborhood of a nuclear warhead. That's not an existential threat or an immediate one or a likely one, but it bloody well is a threat, and it needs to be dealt with, as do the nuclear arsenals of Pakistan and India, and Israel.
It's long past time for negotiations of a peace treaty ending the Korean War, one that prohibits nuclear weapons on or near the Korean Peninsula. It is also time for ensuring a nonnuclear Arabian Peninsula with cutbacks on Israeli bombs, and scaling back of nukes on the Indian subcontinent — and a renewed dedication to nuclear nonproliferation planet-wide so that the increased safety of scale-backs isn't negated.
Here's a couple Old Rules! for you. (1) From writing courses: Cut modifiers as much as possible; let nouns and verbs do the work. (2) Mass murder doesn't have to be genocide to be a horrible act; threats don't have to be existential to be serious. So let's say, North Korean development of nuclear warheads and missiles to deliver them is a threat to their neighbors and to the United State; the American government should lead the way dealing with that threat, and use it as an occasion to work on even more horrific threats.