These eventualities are expected to be at the heart of an extraordinary session here in Brussels on Thursday, when President Biden is meeting with the leaders of the other 29 NATO nations, who will meet for the first time — behind closed doors, their cellphones and aides banned — since Mr Putin invaded Ukraine.

Just a month ago, such scenarios seemed more theoretical. But today, from the White House to NATO headquarters in Brussels, a recognition has taken hold that Russia could turn to the most potent weapons in its arsenal to extricate itself from a military stalemate.

NATO Secretary General Jens Stoltenberg underscored the urgency of the preparedness effort on Wednesday, telling reporters for the first time that even if the Russians employ weapons of mass destruction only inside Ukraine, they can have “disastrous consequences” for the inhabitants of NATO countries. He appeared to be discussing the fear of chemical or radioactive clouds drifting over the border. One question under consideration is whether such collateral damage would be considered an “attack” on NATO under its charter, which may require a joint military response.

The current team was established in a memo signed by Jake Sullivan, Mr. Biden’s national security adviser, on February 28, four days after the start of the invasion, according to officials involved in the process, who have spoke on condition of anonymity to discuss. sensible planning. A previous iteration had worked for months, behind the scenes, to prepare the US government for the likelihood of a Russian invasion of Ukraine.

This team played a pivotal role in designing the deep sanctions playbooks, building up troops in NATO countries, and arming the Ukrainian military, which exploited Russian weaknesses and put its government and economy under enormous pressure.

Mr Stoltenberg, sounding much more belligerent than in the past, said he expected ‘allies to agree to provide additional support, including cybersecurity assistance and equipment to help Ukraine protect against chemical, biological, radiological and nuclear threats”.

As Mr. Biden flew to Europe on Wednesday, he and Mr. Stoltenberg warned of mounting evidence that Russia was in fact preparing to use chemical weapons in Ukraine.

These are questions Europe has not faced since the depths of the Cold War, when NATO had far fewer members and Western Europe worried about a Soviet attack on Germany. . But few of the leaders who were due to meet in Brussels on Thursday never had to deal with those scenarios – and many never had to think about nuclear deterrence or the effects of detonating battlefield nuclear weapons, designed to be less powerful than those that destroyed Hiroshima. The fear is that Russia is more likely to use these weapons precisely because they erode the distinction between conventional and nuclear weapons.

Sen. Jack Reed, a Rhode Island Democrat who heads the Armed Services Committee, said Wednesday that if Mr. Putin used a weapon of mass destruction – chemical, biological or nuclear – “there would be consequences” even if the The weapon’s use was limited to Ukraine. Mr Reed said radiation from a nuclear weapon, for example, could spread to a neighboring NATO country and be seen as an attack on a NATO member.

“It will be a very difficult call, but it is a call that not only the president but the entire NATO Council will have to make,” Mr Reed told reporters, referring to the NATO governing body. western alliance.

“The bottom line is that this is a NATO decision,” Reed said. “It will not be the decision of the president alone. I don’t think he would want to act unilaterally.

One of the main issues that Tiger Team is looking at is the threshold that might incentivize the alliance to use military force in Ukraine. Mr Biden has made it clear that he is extremely reluctant to do so, fearing that a direct confrontation with Russia will escalate the conflict beyond control. “It’s World War III,” he noted recently.

A second team of officials, also created by Mr. Sullivan’s February 28 memo, is studying long-term opportunities for the United States to improve its geopolitical position in the wake of Mr. Putin’s invasion. Inside the White House, it has become an article of faith that the Russian leader made a huge strategic mistake – a mistake that will diminish Russia’s standing, cripple its economy and alienate potential allies for years to come. But it is early in the conflict, other officials warn, and that conclusion may prove premature.

The immediate concern is what Mr Putin might do next – driven by a desire to salvage a faltering military effort or restore his credentials as a force to be feared.

Officials believe the chances of Mr Putin resorting to detonating a nuclear weapon are slim. But the constant stream of reminders from Russia that it has its arsenal ready and could use it in response to anything it perceives as an “existential threat,” has Washington on high alert.

Mr. Biden will discuss with his allies “how to deal with the rhetoric and the comments coming out of Russia on this whole issue of the potential use of nuclear weapons,” Mr. Sullivan told reporters on Wednesday.

“We haven’t seen anything that has caused us to adjust our posture, our nuclear posture, but this is of course something on which we will have to continue to remain in close consultation with our allies and partners, as well as communicate directly with Russians.”

Several officials have said the White House and Pentagon have had tension over how much detail the Department of Defense is willing to share about its top-secret war planning – particularly regarding responses to any use of nuclear weapons. – even within the classified framework of the Tiger team. (The term has been used for many years to describe an emergency task force within the National Security Council.)

A US official said Mr Biden remained adamant about keeping US forces out of Ukraine. But the official said the administration believes it would be wrong not to look closely at the thresholds, if any, below which the president would roll over, or to be prepared to deal with the consequences of using of weapons of mass destruction.

A senior administration official said any use of a “small” tactical nuclear bomb by Russia – even inside Ukraine and not directed at a NATO member – would mean that “all paris are open” on the fact that the United States and NATO stay out of the war. But when pushed, the official refused to present the answers for discussion.

The official said the US and NATO intelligence communities had seen no activity from Russian military officials suggesting preparations to use a nuclear weapon. But he said in internal discussions, administration officials were urging caution because there was more at stake than Ukraine.

If Mr. Putin intentionally hit a NATO country, not only would he bring the force of the military alliance to bear on Russia, but he would also likely come face to face with NATO troops inside Ukraine. , Latvian Defense Minister Artis Pabriks told journalists traveling in his country. country this month with Gen. Mark A. Milley, the Chairman of the Joint Chiefs of Staff.

“He will get Article 5,” Pabriks said, referring to NATO’s pledge that an attack on one member of the alliance is an attack on all.

“If he gets that, it would involve us in Ukraine as well,” Mr Pabriks said, adding: “He has no way out. So I don’t think he should be that stupid.

Senator Angus King of Maine, an independent and member of the Senate Intelligence and Armed Services Committees, traveled to the Polish-Ukrainian border over the weekend, meeting with officials from allied countries, touring a refugees and chatting with Ukrainians. Mr King said that as Russian forces struggled to advance, Mr Putin could try to strike a diplomatic deal, step up his bombardment of Ukrainian towns and level them, or go after the West with a cyberattack.

“The fourth is to step up to defuse, which is a tactical nuke,” Mr King said, using the term from a Russian military doctrine in which he would employ a nuke as a warning – then negotiate.

David E. Sanger reported from Brussels, and Eric Schmitt, Helen Cooper and Julian Barnes from Washington.