In the rarest eclipse imaginable, Joe Biden and Donald Trump are in alignment on one thing: Russia’s invasion of Ukraine shouldn’t be allowed to pull us into World War III. Sen. Marco Rubio has chipped in the same portentous thought.

Beware wars to come is always a useful idea, but let’s hope America’s political leadership has a clearer view of what is happening in the here and now. Ukraine’s President Volodymyr Zelensky does. Asked by NBC News Tuesday if he understood Mr. Biden’s concerns about World War III, replied: “”Nobody knows whether it may have already started.”

Vladimir Putin’s invasion of Ukraine has already produced World War 2½.

World War III is a decades-long Cold War concern. What we’re seeing now is something new—a spontaneous, crowd-funded world-wide effort to fight a war without springing the Armageddon tripwire.

Within three weeks, virtually all the world’s democratic nations and peoples have committed to repelling Russia’s invasion. The size, speed and depth of the response is almost incomprehensible.

Countries that have sent modern weaponry or military equipment to Ukraine include not only the U.S. but also the U.K., Germany, France, Canada, Belgium, the Netherlands, Sweden, the Czech Republic, Australia, Portugal, Romania, Denmark, Croatia, Slovenia, Italy and Japan. South Korea said Tuesday it will send nearly $1 billion of nonlethal equipment.

Responding to Mr. Zelensky’s Feb. 27 call for an International Legion of Defense of Ukraine, thousands of volunteer fighters, some combat veterans of Iraq and Afghanistan, are pouring in from everywhere.

International boycotts have made Russia a corporate, financial and cultural pariah. Overnight, the world’s best nonprofits created an infrastructure of support in Ukraine or on its border.

The post-Cold-War era has never seen anything like what has come to be called “Ukraine,” and it will take time to absorb its meaning and effects.

For starters, those of us who labor in the trenches of the culture wars tend to think tech and its media platforms on balance are corrosive and toxic. And they are. But we need to acknowledge that those same messaging forces—which quickly assemble group thinking on a massive scale—here put in motion something powerful and potentially good.

Will the response stop Mr. Putin? Who knows? But it might, if we posit that the solid building block for this broad effort to resist is Ukraine itself—the unexpected ability of the country’s military to counter the Putin blitzkrieg and its people’s refusal to surrender.

Modern media like YouTube and Instagram helps create tissue-thin celebrities and “influencers.” Here the platforms instantly elevated Mr. Zelensky, a professional comedian, into a heroic influencer motivating his own people and tens of millions outside his country. When is the last time every member of the U.S. Congress stood to applaud a president?

The world’s behavior suggests, for the moment anyway, that we’ve decided that we’re all the North Atlantic Treaty Organization now, bound by a kind of global Article 5: A massive unprovoked attack on one of us is an attack on all of us.

Ukraine has put in motion two human impulses: to help and to defend. The first, to help, is a constant in our times. The second, to defend, is an impulse many of us thought had died, as with Germany’s miniaturization of its military capacity.

The help-Ukraine piece emerges paradoxically from the often reflexive, do-good sentiments that seem on tap everywhere today, such as the unfocused virtuousness of “save the planet.”

With “Ukraine,” we seem to have arrived at an unlikely conjunction of virtue signaling and genuine virtue. The outpouring for Ukraine is the real thing. Perhaps many all along were seeking an authentic outlet for their better spirits, rather than the constant manufactured appeals—and lately coercion—to be on the “right side” of marginal causes like fur or pronouns. This flight from faux benevolence may not last, but for now it’s all good.

Ukraine’s more enduring effect could be to re-establish the legitimacy of national—and personal—self-defense. Extending baby-land into adulthood is over.

Mr. Putin’s indiscriminate bombing of civilians has restored a historical reality: Well-armed political messianists mean what they say, whether in Moscow, Tehran, Beijing or Pyongyang.

Defense against such threats has two forms: pre-emptive strikes, meaning war, or deterrence. The better default is deterrence.

Deterrence requires a willingness to spend national wealth on defense, as Ukraine chose eight years ago—as NATO, it appears, is willing to do now. Congress in the past week more than doubled Mr. Biden’s proposed percentage increase in the U.S. defense budget. Two months ago, Australia and Japan signed a defense agreement. “Ukraine” has leveled the field of debate between spending on national security and domestic needs.

Invocations of World War III by public figures as prominent as Messrs. Biden and Trump sound like excuses now to revert later to the passive pre-Ukraine status quo. The global support of “Ukraine”—a recognition that suddenly we are in World War 2½, with tough choices ahead—suggests people will be looking for much more than the Cold War status quo from their national leaders.

Write henninger@wsj.com.

This column has been updated to cite President Zelensky’s interview with NBC News.