For weeks, officials in Washington have been supremely cautious in discussing whether President Putin might use a low-yield tactical nuclear weapon in Ukraine.
President Biden last night abandoned all such moderate language and issued by far the most alarming assessment of the risk of a disastrous escalation in the war.
At a private event in New York, Biden appeared to suggest that the Russian leader might detonate a nuclear weapon in Ukraine, declaring: "We have not faced the prospect of Armageddon since Kennedy and the Cuban missile crisis."
He said that Putin is "not joking when he talks about potential use of tactical nuclear weapons or biological or chemical weapons because his military is... significantly underperforming."
By comparing the current situation to the terrifying Cuban missile crisis of 1962 - widely seen as the closest the world has come to a superpower nuclear exchange - Biden has just sent shockwaves through Washington.
It is possible that Biden is getting far ahead of intelligence assessments. It could have been a gaffe or an exaggeration - to which, of course, he is prone.
But when a US commander-in-chief even talks of a possible Armageddon scenario, it is of grave concern to everyone.
Other US officials have been careful with their language.
Want a quick and expert briefing on the biggest news stories? Listen to our latest podcasts to find out What You Need To Know...
While acknowledging that Russian forces are facing major battlefield reversals, and may threaten nuclear escalation, the Pentagon has always been quick to point out that there is no sign of any change to the alert status or posture of Russia's nuclear forces.
And if you thought that American politics would avoid making a nuclear danger into a partisan political point, you are mistaken.
Donald Trump Jr, the son of the former president, tweeted out last night his concern about Biden's abilities:
In any case, President Biden has at a stroke raised the levels of concern about Putin's intentions.
It is a stark reminder that a wounded and humiliated Russian leader does indeed have one final, catastrophic option.