The headline in this morning’s Kommersant newspaper captured the drama.

“Vladimir Putin draws his red line.”

Will the West cross it? And, if it does, how will Russia respond?

Speaking in St Petersburg, President Putin sent a clear warning to the West: don’t allow Ukraine to use your long-range missiles to strike Russian territory.

Moscow, he said, would view that as the “direct participation” of Nato countries in the war in Ukraine.

Russia has drawn red lines before. And seen them crossed before.

On 24 February 2022, when he announced the start of his "special military operation" – the full-scale invasion of Ukraine – President Putin issued a warning to “those who may be tempted to interfere from the outside”.

“No matter who tries to stand in our way or create threats for our country and our people, they must know that Russia will respond immediately,” the Kremlin leader had declared.

“And the consequences will be such as you have never seen in your entire history.”

Western leaders ignored what was widely interpreted at the time as nuclear sabre-rattling. The West has since provided Ukraine with tanks, advanced missile systems and, most recently, F-16 American fighter jets.

This year Russia has already accused Ukraine of using American long-range ATACMS missiles to target Crimea, the Ukrainian peninsula annexed by Russia.

What’s more, over the last two years, Russian officials and the state media here have on many occasions accused the West of “fighting Russia” or launching “a war” on Russia. Even though it was Russia that invaded Ukraine.

But from the tone of President Putin’s latest remarks, it’s clear he considers that the targeting of internationally recognised Russian territory with Western missile systems would take the conflict to a new level.

What he didn’t make clear yesterday is how Moscow would respond.

“We will take corresponding decisions based on the threats to us that will be created,” Vladimir Putin said.

On Friday, Russia withdrew the accreditation of six British diplomats, accusing them of “subversive activities” and threatening Russia’s security.

But Putin’s potential response is much broader. He offered some clues back in June.

At a meeting with the heads of international news agencies, he was asked: how would Russia react if Ukraine was given the opportunity to hit targets on Russian territory with weapons supplied by Europe?

“First, we will, of course, improve our air defence systems. We will be destroying their missiles,” President Putin replied.

“Second, we believe that if someone is thinking it is possible to supply such weapons to a war zone to strike our territory and create problems for us, why can’t we supply our weapons of the same class to those regions around the world where they will target sensitive facilities of the countries that are doing this to Russia?”

In other words, arming Western adversaries to strike Western targets abroad is something that Moscow has been considering.

Earlier this month, Russia’s deputy foreign minister, Sergei Ryabkov, announced that Russia was set to revise its nuclear doctrine: the document that lays out under what circumstances Moscow may consider using nuclear weapons.

He suggested that the decision to revise the doctrine was “connected with the escalation course of [Russia’s] Western adversaries”.