It’s human nature to avoid — and deny when possible — messages that we don’t want to hear.

My wife Peggy, a TWA flight attendant (they were called stewardesses then) when we first met in 1968, has often recounted a perfect example from her flying days. The TWA fleet at that time included a passenger jet called the Convair 880. One of the 880’s idiosyncrasies (there were many) was a failure-prone emergency oxygen system, the vital safety equipment that makes it possible for passengers to breathe in the event of cabin depressurization. All air travelers have heard the pre-flight safety instructions: “If oxygen is needed, the masks will automatically drop from the overhead bin. If so, place your mask over your nose and mouth and breathe normally.”

The problem was that on the Convair 880s, the system would occasionally malfunction and the masks would drop for no reason. Peggy recalls that whenever that happened, people would fidget awkwardly, trying their best to ignore the oxygen masks dangling right in front of them — and no one would ever put them on as briefed. Evidently, their instinctive psychological reaction was, “No thanks, I don’t want to be in a plane crash right now.”

With war in the Ukraine, we — the U.S. and our European partners — are reacting the same way. World War III? Nuclear attack? No thanks, not right now. Clearly, that’s too scary a specter to confront.

We’re attending to the business-as-usual matters that crop up in international disputes and natural disasters. We’re ready to protect our NATO partners, as if that were the only thing that could push us into a direct conflict. We’re supporting magnificent public and private initiatives to help the Ukrainian victims — food, supplies, medicine, money, anything to salve their wounds — but doing precious little to protect them from their aggressors.

We continue to hope for a negotiated settlement, as if one could ever reach satisfactory agreement with a vicious killer. And — illogically — we are far too concerned about “provoking” Vladimir Putin, despite his obvious willingness to mete out death and destruction with no provocation at all.

Under other circumstances, these would be sensible courses of action. But in this case, they ignore the monstrous reality of what is happening before our eyes: Russia is laying waste to another sovereign nation, destroying cities, murdering countless civilians, producing millions of refugees, creating and international emergency of the highest order, and very possibly kick-starting the most destructive war in world history.

Militarily, Russia’s invasion of the Ukraine has been unimpressive. But Putin has one finger on the nuclear button, and everyone knows it. In effect, the rest of the free world is now frozen into inaction, hoping that somehow the problem will go away.

The barbarians are at the gates — and they’ve got nukes.

Since the dawn of the nuclear age, mankind has struggled with the monumental problem of dealing with the very existence of weapons powerful enough to destroy the planet and everyone on it.

For 75 years, a central element of our defense strategy — and our best hope in avoiding confrontation with another nuclear superpower — has been the tenuous standoff called “mutually assured destruction” (with the ironic acronym MAD). It’s the idea that no one would ever launch a nuclear weapon because the attack and the response to it would destroy both aggressor and victim.

But now nuclear confrontation is here, and it’s taking a somewhat different form: nuclear brinksmanship exercised by a barbaric assailant, who is in effect saying to his adversaries, “Watch me, I can do whatever I want, take what I please, abuse whomever I choose, because you know I’m ruthless enough to pull the trigger.”

I’m no military strategist, but the underlying situation seems very clear. We simply must confront the reality of a nuclear-armed bully who is evidently confident that no one will stand in his way.

Whatever advantage just a few months ago was lost when we and our international allies failed to deter Putin from invading the Ukraine. We learned the hard way that he’s impervious to threats of future dire consequences. Evidently, he understands only present, real constraints.

We must strangle him economically, now, and we must interdict his ability to resupply and rearm his deployed forces, in both ways rendering it impossible for him to continue his invasion. He will surely consider such actions provocative, and that poses risk, but at this point we have no viable alternative.

Going forward, whatever steps we take to stop Putin — including military actions — are essentially defensive in nature, intended not to make war but to protect Ukraine and the rest of the world from a rampaging nuclear barbarian.