BRUSSELS — As President Biden and 29 other leaders of NATO walked into the alliance’s sprawling Brussels headquarters Thursday morning, they passed a graffiti-sprayed remnant of the Berlin Wall, a monument to Europe’s belief that it had won a permanent victory over the nuclear-armed, authoritarian adversary that challenged the West throughout the Cold War.
Now, exactly one month into the Russian invasion of Ukraine, the mood permeating the extraordinary NATO summit meeting was a mix of both fear and opportunity.
The fear is that the aftermath of the invasion has rapidly transformed Europe into two armed camps once again, though this time the Iron Curtain looks very different. The opportunity is that, 30 days into a misbegotten war, Russia has already made so many mistakes that some of the NATO leaders believe that, if the West plays the next phase right, President Vladimir V. Putin of Russia may fail at his apparent objective of taking all of Ukraine.
That does not mean the Ukrainians will win. Their country is shattered, millions are dispersed and homeless, and among leaders who gathered in Brussels there was a sense of foreboding that the scenes of destruction and violence could go on for months or years. No one saw an outcome in which Mr. Putin would withdraw. Instead, there was concern he could double down, reaching for chemical, or even tactical nuclear, weapons.
But there was a surprising tenacity about taking on Mr. Putin — a sense that did not exist broadly across Europe until the invasion began, and that has only intensified since.
“I don’t think we have any choice,’’ Roberta Metsola, the president of the European Parliament, said as Mr. Biden moved from NATO headquarters to the headquarters of the European Union in his day of emergency meetings. “We know that any indecision or any differences will be exploited by Putin and his allies.”
Twice during the series of meetings, President Volodymyr Zelensky of Ukraine beamed in from his command post in Kyiv, telling the others that no matter how proud they are of how they have stood up to Mr. Putin, they have not done enough. Ukraine, he suggested, was fighting a war for Europe — and one that Europeans, as much as Ukrainians, could not afford to lose, because Mr. Putin would not stop at Ukraine’s borders.
Mr. Zelensky reminded them that a month ago — to the day — “I addressed you with a perfectly clear, logical request to help close our skies. In any format. Protect our people from Russian bombs and missiles.”
But “we did not hear a clear answer,” he said, not sugarcoating his critique. “And you see the consequences today — how many people were killed, how many peaceful cities were destroyed.”
The meeting was Mr. Biden’s idea, and it took some European diplomats by surprise, because they had to quickly devise initiatives — from new sanctions to a declaration that they would provide chemical and biological protection equipment to Ukraine — to signal that they did not just talk about a problem.
Mr. Biden, speaking to reporters later, said his real purpose was to ensure that the pressure he has built against Russia does not fade.
“Look, if you are Putin, and you think that Europe is going to crack in a month, or six weeks, two months — they can take anything for another month,” Mr. Biden said. But he said “the reason I asked for the meeting is we have to stay fully, fully, fully” agreed on constant pressure.
He even suggested expelling Russia from the Group of 20 industrial economies, an organization that encompasses China, among others, and mixes democracies and authoritarian states. Even if Russia could not be removed, he suggested, Ukraine should be added to the meetings, a move that would enrage Mr. Putin.
Yet it is the early success of that pressure campaign that is also creating the danger.
While the ostensible purpose of the sanctions is to force Mr. Putin to withdraw from Ukraine, no leader who spoke on the edges of the meeting sounded as if there was much confidence that would happen. Quite the opposite: the concern permeating NATO is that frustration, isolation and international criticism will prompt Mr. Putin to intensify the war.
That is why so much time was spent inside NATO headquarters debating how NATO might respond to an escalation — especially the use of chemical weapons, perhaps to force Mr. Zelensky to abandon Kyiv, the capital. Mr. Biden, asked repeatedly after the meeting about that response, dodged the question.
Jens Stoltenberg, the NATO secretary general, has over the past two days described the urgent need to provide protective gear to the Ukrainians, and he said that NATO nations would go on high alert for any atmospheric signals that chemical weapons are being released.
Russia-Ukraine War: Key Developments
“Our top military commander General Wolters has activated NATO’s chemical, biological, radiological and nuclear defense elements,’’ Mr. Stoltenberg told reporters, referring to Gen. Tod D. Wolters, NATO’s Supreme Allied Commander, Europe, “and allies are deploying additional chemical and biological and nuclear defenses.” That has not happened on this scale in modern memory, military experts say.
Officials will not say what intelligence underlies the warnings that Mr. Putin might now turn to unconventional weapons — other than the reality that he has done so before, against exiled spies and dissidents. And the possibility is being discussed in public to deter Mr. Putin from acting.
Few anticipated this danger just a month ago. Then again, most assumptions from mid-February have crumbled.
Before the invasion, NATO officials assumed the Russians were unstoppable, that they would surge across Ukraine in 30 days, seizing the southeast and the capital, according to their own war plan. Now, while few believe that Ukrainian forces can win, there is a widespread assumption that they might fight Russia to a stalemate — stopping its advances around the capital.
The accepted preinvasion wisdom in Washington and some European capitals was that Mr. Putin was a master tactician, and that he had “sanction-proofed” his economy. Today it is clear he left himself highly vulnerable, and is surviving on one major revenue stream: Europe’s addiction to Russian fossil fuel, the one import the continent has declined to block so far.
A month ago President Biden’s talk of making democracy prevail over autocracy seemed like a gauzy ideological sheen surrounding his plans to take on China. Today, as Mr. Biden got the leaders to endorse a new program to bolster other fragile democratic states worried that they will be in Mr. Putin’s cross hairs next, it has a different meaning.
Mary K. Brooks contributed research.