“Many new weapons systems were deployed to the Far East first, even though Russia always said NATO was the main threat and China was a partner,” he said.

Russia and China have a dualistic history – one moment friends, the next moment enemies, then switching back again. They were united in Communist solidarity early in the Cold War, then plunged into the Sino-Soviet split.

In 1969, Chinese forces ambushed Russia over a disputed island and the two nations fought a furious series of border skirmishes lasting half a year. Moscow considered a nuclear attack on China, which also had nuclear weapons by that stage. It might have descended into nuclear war and the situation was so tense that Mao Zedong ordered the evacuation of Beijing at one point.

Vladimir Putin and Xi Jinping signed their so-called “no limits” partnership two years ago. China has supported Russia economically and diplomatically in its war on Ukraine. But it has kept some distance – Beijing has declined to send military aid openly to Russia.

The second fascinating revelation from the leaked Russian files is Moscow’s apparent readiness to use nuclear weapons. “They show that Russia’s declaratory policy doesn’t depict fully how they might use nuclear weapons at levels below the declared threshold,” US strategy expert Elbridge Colby tells me.

Loading

“If you think Putin is an evil guy, this is all the more reason to take him seriously” when he threatens to use nuclear bombs in Ukraine or against NATO, something he did as recently as last week.

A confrontation “could escalate very quickly across the nuclear threshold, not end-of-the-world stuff but to give a face shot or to grab the enemy by the lapels”, says Colby, lead author of the US national security policy produced for the Trump administration.

This is a vital distinction. Strategic weapons are designed for mass annihilation of cities and major targets; tactical ones are smaller-scale bombs intended for battlefield use.

Among the Financial Times′ leaks, it quotes secret “broader criteria for a potential nuclear strike, including an enemy landing on Russian territory, the defeat of units responsible for securing border areas, or an imminent enemy attack using conventional weapons”.

Loading

A secret briefing for Russian navy officers canvasses the threshold, including “the destruction of 20 per cent of Russia’s strategic ballistic missile submarines, 30 per cent of its nuclear-powered attack submarines, three or more cruisers, three airfields, or a simultaneous hit on main and reserve coastal command centres.”

At the moment, while Russia appears to have the advantage in its war on Ukraine, Colby thinks that Putin is most unlikely to resort to nuclear force. And, even if he did, Putin would risk having an underwhelming effect, says Colby: “I think they tend to exaggerate the battlefield consequences of relatively low-yield tactical nuclear weapons. If he does it and it doesn’t work, it’s not a high payoff.” He points out that Russia has not tested its weapons in many years.

As for China, Xi gave powerful tacit endorsement of Russia’s war by visiting Moscow last year. As Putin bid him farewell and shook his hand, Xi told the Russian president: “Right now there are changes, the likes of which we haven’t seen for 100 years. And we are the ones driving these changes together.”

The two men are partners until the day they are not. Says Dibb: “The danger for Russia is that as it becomes the junior ally, it will find that hard to swallow.” Even last year, in the midst of their supposed intimacy, Russia twice tested its nuclear-capable Iskander missile in areas bordering China.

Paranoid dictators deserve each other.

Peter Hartcher is international editor.