Included in the documents is info on the Kremlin's minimum criteria for using tactical nukes.

The criteria outlined in the secret files range "from an enemy incursion on Russian territory to more specific triggers, such as the destruction of 20% of Russia's strategic ballistic missile submarines," according to FT.

The question of the possibility of Russia and NATO stumbling toward nuclear war is without doubt heavy on the minds of many this week, especially after French President Emmanuel Macron's Monday comments wherein he raised sending Western troops to fight Russia in Ukraine and said of the possibility, "nothing should be ruled out."

Putin appeared to respond directly in his Thursday state of the nation televised address, spelling out: "Everything that they are coming up with now, with which they threaten the entire world – all this really threatens a conflict with the use of nuclear weapons, and therefore the destruction of civilization – don't they understand this, or what?"

"They must ultimately understand that we also have weapons – and they know about it, just as I now said  – we also have weapons that can hit targets on their territory," he warned.

As for tactical nuke usage, Russia's tactical arsenal has more limited range in comparison to strategic weapons, and thus are designed and intended for the possibility of a 'nearer' war in Europe or Asia. 

But worrisomely, the FT review of the documents (which date from 2008-2014) finds that the Kremlin likely has a threshold "lower than Russia has ever publicly admitted, according to experts who reviewed and verified the documents." Experts cited in the FT say the contents of the leaks likely remain part of Moscow's current nuclear doctrine.

The documents show that Russia has recently rehearsed scenarios involving war with China. Per the leaks and the FT report:

One exercise outlining a hypothetical attack by China notes that Russia, dubbed the "Northern Federation" for the purpose of the war game, could respond with a tactical nuclear strike in order to stop "the South" from advancing with a second wave of invading forces.