Vladimir Putin’s dangerously irresponsible move to put Russia’s nuclear arsenal on “high alert” triggered chilling memories for those of us who grew up during the dark days of the Cold War.

Huddling under tiny desks in duck-and-cover drills, we quickly learned in elementary school that our very lives dangled from a fraying nuclear cord. The 1962 Cuban missile crisis — which could have ended with nuclear misadventure — reminded us of the mushroom-cloud dangers of unstable leaders.

It is hard to describe those never-quite-erased fears of nuclear holocaust to those too young to recall, say, Ronald Reagan telling the leader of the Soviet Union in 1987, “Mr. Gorbachev, tear down this wall.”

Putin playing the nuclear card is more than just hyperbolic rhetoric. It is a reminder of how miraculously lucky the world has been that no nuclear weapon has been used in wartime since 1945.

Despite the valid fears of a NATO military confrontation with nuclear-armed Russia, it has been uplifting how much the world has rallied to, at least, emotionally support Ukraine. I am far from alone in my new habit of waking up in the middle of the night to double-check on the status of Kyiv and Volodymyr Zelenskyy.

Of course, admiring the courage of average Ukrainians from a place of safety seven time zones to the west of Kyiv is a no-risk enterprise, like tearing up during a World War II movie on TV. But, like World War II, there is something bracing in the near universal recognition of the stark good-vs.-evil nature of the current conflict.

We have witnessed other heart-rending tragedies over the years — ethnic cleansing in Bosnia, Bashar Assad’s war on his own people in Syria and the brutal anti-democratic crackdown in Hong Kong. Not to mention America’s own blundering in Iraq and Afghanistan.

But my sense is that the Ukraine war represents a different kind of moment when political attitudes change overnight — and stay that way.

Already, Republicans are afflicted with amnesia as they wipe from memory any words that ever supported Donald Trump’s embrace of Putin. Many in the GOP have also become curiously forgetful about defending the former president’s “perfect” call to Zelenskyy as Trump tried to trade congressionally authorized weaponry for dirt on Hunter Biden.

Events in Ukraine have mostly silenced the isolationist calls for America to turn its back on NATO as a relic of a bygone era. Once again, there is, sadly, a powerful rationale for a trans-Atlantic alliance to counter Russian aggression.

In his State of the Union address on Tuesday, I hope Joe Biden explains in clear language the obligations that America and its allies shoulder under Article 5 of the North Atlantic Treaty. That article states that “an armed attack” against one NATO nation “shall be considered an attack against all of them.”

Ukraine, of course, is not a member of NATO. But the Baltic states (Estonia, Latvia and Lithuania), all of which border Russia, are in NATO. And, more ominously for their futures, all three tiny democracies were forcibly part of the Soviet Union from World War II until they declared their independence in 1991.

The Baltic states are obviously part of Putin’s mad-ruler dreams of reestablishing the Soviet empire. Any Russian military challenge to these NATO nations would have to be countered — under Article 5 — with allied forces and not just economic sanctions.

If Russia is allowed to prevail in Ukraine, the Baltics may well be next. And that would confront America and NATO with what they have been desperately trying to avoid — a direct military faceoff with a nuclear adversary.

Up to now, most Americans have had the luxury of ignoring most aspects of foreign policy unless U.S. troops were on the line. But everything changed as soon as Russian troops crossed the Ukrainian border. Suddenly, we are as enmeshed in Europe as we were during the Cold War.

That is why I hope that Biden devotes a significant portion of the State of the Union to a tutorial about what this dangerous world that Putin is creating means for America.

Achieving lasting national unity in a polarized political era against a powerful adversary like Russia will not be easy. But the task will be far more challenging if Biden does not begin to lay the groundwork in what may prove to be the most important speech of his presidency.

Russian military incompetence and the stirring resolve of the Ukrainians have bought the world a few days, a week, or, at best, a month to prevent the annihilation of an independent democratic nation.

Maybe Russia can be forced to withdraw by the withering attack on its financial stability. Maybe a turning point will come when oligarchs have to explain to their teenage children why they now have the limited choice of college in Moscow or Minsk.

But the real test is how much military aid Americans and Europeans can funnel to the Ukrainian forces before it becomes too late. Already, we have passed the point when congressionally mandated sanctions against Russia have ceased to matter except at the margins.

“It’s your fight, too” is an old American political slogan used by former House Minority Leader Richard Gephardt in his failed 1988 quest for the Democratic presidential nomination.

But repurposed, those words also convey America’s stake in the battle for Ukraine. It’s our fight, too. Either the Russians are turned back at the gates of Kyiv or else the next war launched by Putin will be against our NATO allies.