It was 2008, on the eve of the Bucharest Summit, where the North Atlantic Treaty Organization considered expanding by inviting new members from the Balkans and the former Soviet Union. André Glucksmann and I co-signed an open letter to the French president and the German chancellor urging them to hear the pleas of Ukraine, which had sought to protect itself from the Russian empire since it declared independence in 1918.

Seventeen years after its second emancipation in 1991, leaders in Kyiv saw no other path than this one: the Membership Action Plan, which would allow their nation one day to join NATO.

We argued that the nascent 21st century wasn’t so favorable to the West that we could so easily reject a country that, at its own peril, was undertaking to join our institutions. We explained that lowering the flag in the face of the Kremlin’s pressure and blackmail, curling up in a ball and closing our door, two decades after the fall of the Berlin Wall, to the last of the voices of the captive Europe, would be a grave political and historical error.

I spent the following years—with Glucksmann until his death in 2015—repeating what we’d learned from the two Chechen wars, the invasion of Georgia and the testimony of the early dissidents: that Vladimir Putin has one main enemy he wants to bring to heel, the Europe of democracy, rule of law and free republics.

In the decade and a half since, the West gave Moscow every pledge of good faith it demanded and more. France sold Russia warships. President Obama signed an arms treaty that put Russia at an advantage and canceled the deployment of antimissile defenses in Poland and the Czech Republic, as Mr. Putin demanded. President Trump treated Mr. Putin with a sickening solicitude. The allies never stopped reassuring the Kremlin of the solidity of the 1990 Paris Charter, the 1994 Partnership for Peace, the 1999 Charter for European Security and the 2002 NATO-Russia Council.

Far from being scorned, isolated and humiliated, Russia was treated with an astonishing and unwarranted respect. Mr. Putin, taking these gestures as admissions of Western weakness, concluded that he could attack Ukraine. Because Ukraine wasn’t part of NATO, we found ourselves on the brink of a global conflict. Hoping to avoid war by betraying a friendly nation, we ended up, as always, with dishonor and war.

What now? The answer is obvious.

It’s the one Henry Kissinger came to last week in Davos, where he said that his principal argument against Ukrainian NATO membership—not provoking Russia—has fallen, and he now thinks the idea of a neutral Ukraine “is no longer meaningful.”

It’s the one the Ukrainian defense minister, Oleksii Reznikov, came to: Since his army fights with our weapons, our munitions and now our tanks, it has, out of necessity, adopted the norms of the alliance and become a de facto member.

For the past 11 months, I have accompanied, listened to and filmed the Ukrainian troops in action. These brave fighters are like the Athenian soldiers Pericles celebrated in his address to heroes as described by Thucydides. They fight for their country but also in defense of the higher values of liberty, democracy, rights—in short, Europe.

Are they not our champions? Our bulwark against the common enemy? Do they not defend us, in their trenches, as much as we defend them? Have they not become, by force of circumstance, the most seasoned, the most experienced, the best of the armies of Europe? And this army—which will soon operate our Leclerc, Abrams and Leopard tanks—doesn’t it possess a strategic and tactical know-how that, in the coming troubled times, will prove invaluable?

Bringing Ukraine into NATO is as much in our own strategic interest as it is our moral duty. Accelerating the process to make up for time lost since 2008 is, for all parties, a matter of security as much as dignity. The sooner it happens, the sooner peace will return, a real peace that can be achieved only through the total capitulation of Mr. Putin, his regime and his army.