Facing increasing political pressure at home, isolation abroad and battlefield humiliations, the Russian leader ratcheted up his nuclear brinkmanship last week in warning that he could use all weapons systems available to him if he considered Russia's territorial integrity under threat.
But White House national security adviser Jake Sullivan issued an ominous public caution to Putin on Sunday.
"If Russia crosses this line, there will be catastrophic consequences for Russia. The United States will respond decisively," Sullivan said on NBC's "Meet the Press." He added that in private channels, the US warning had been more stark but declined to draw red lines in order to keep such contacts open and to avoid "a rhetorical tit-for-tat."
Secretary of State Antony Blinken reinforced that message on CBS' "60 Minutes" in another sign that Washington is increasingly adding a public element to its private pressure on the Kremlin on this issue.
"It's very important that Moscow hear from us and know from us that the consequences would be horrific," Blinken said.
Putin's rhetoric was a reminder that the better the war goes for Ukraine, the more the West will need to keep its nerve, especially if the Russian leader becomes more boxed in and tries to scare his foes with Russia's best leverage -- its nuclear arsenal.
Many Western observers believe Putin is bluffing and that there are strategic reasons for Moscow to stop short of this fateful step. There are no public reports that the Kremlin is readying its stock of battlefield nuclear weapons for use or that it has changed the posture of its international strategic missiles. And Putin has played the nuclear card before in the conflict in an apparent effort to frighten Western publics and to fracture support for Kyiv in the transatlantic alliance.
But at the same time, the Russian leader has gone all in on a war that he cannot afford to lose but that is going increasingly badly for Russia, as last week's partial national mobilization showed. He is in a corner, a reality that may explain his return to nuclear scare tactics. And while Putin's political position doesn't seem immediately threatened, he's facing increasing dissent at home and appears consumed by fury against the US and the West that is vehement even for him.
Putin is led by a sense of historic mission rooted in a desire to restore respect for Russia as a great civilization. He has already shown callous indifference to human and civilian life in Ukraine. Such conditions mean clear strategic thinking and rational decisions cannot be taken for granted, especially since the ruthless Russian leader's sense of caution deserted him with his reckless leadership of the war in Ukraine.
And worryingly, Blinken admitted that it remains to be seen whether Russia's nuclear chain of command would work if top military officers wanted to forestall any effort by Putin to use nuclear weapons.
"That is the Achilles' heel of autocracies anywhere -- there is usually not anyone who has the capacity or the will to speak truth to power. And part of the reason, I think, Russia has gotten itself into the mess that it's in is because there is no one in the system to effectively tell Putin he's doing the wrong thing."
A stark US message
It is in this dangerous atmosphere that Washington issued its warning, designed to deter Putin from a cycle of escalation that could raise the risk he might consider, or at least threaten the use, of a limited yield tactical nuclear weapon in Ukraine to compensate for his military's failure in a conventional conflict. The US message also seemed destined for those around the Russian leader, in high-level positions in the military or intelligence agencies, for instance, who may be in a position to influence his thinking or to block his capacity to carry out his threats.
What the catastrophic consequences that Sullivan mentioned would actually be has not been spelled out. But given the magnitude of any use of nuclear weapons, many military and diplomatic experts argue that a response would have to be far stiffer than another round of sanctions on the already debilitated Russian economy. The humanitarian and environmental impact of using even a limited yield nuclear device would surpass the horror and civilian carnage already unleashed on Ukraine. And its usage would also take the world across a dangerous strategic threshold and establish a precedent for the use of nuclear arms to change the equation in conventional conflicts, which could cause a rush by other rogue states to get such a capacity.
That possibility, for now, seems a long way away and would need a lot of things to go wrong and for many off-ramps to be missed. One potential goal of US diplomacy in the immediate term might also be to press on nations like China and India, which still have workable relations with Russia, to convey the kind of global ostracism that Moscow might face if it used its nuclear arsenal.
Still, the spectacle of the President's top foreign policy adviser warning Moscow of the consequences of the use of nuclear weapons amid what is effectively a proxy war in Europe between the West and the Kremlin is a sobering sign of the gravity of the situation.
Reading Putin's mind
Putin's warning that he wasn't bluffing about his willingness to use nuclear weapons if, in his perception, Russia was under attack has set off public and private speculation of what is driving his thinking.
But another European leader who knows Putin well, President Sauli Niinistö of Finland, warned Sunday of a dangerous moment since the Russian leader had now invested so much credibility in a war that has turned against him in recent weeks.
"He has put all in," Niinistö told CNN's Fareed Zakaria.
"He is a fighter, so it is very difficult seeing him accepting any kind of defeat and this surely makes the situation very critical."
The impossibility that Putin -- for historic, personal and political reasons -- would admit he failed in Ukraine has brought the world to a potentially perilous moment.