At its Vilnius summit in July, NATO leaders will adopt hundreds of pages of secret military plans that will explain how the alliance would respond to a Russian assault for the first time since the Cold War.

For decades, NATO saw no need to develop large-scale defense plans while it fought smaller conflicts in Afghanistan and Iraq and was assured post-Soviet Russia no longer constituted an existential threat.

But with Europe’s bloodiest war since 1945 raging just beyond its borders in Ukraine, the alliance is now warning that it must have all planning in place well before a conflict with a peer adversary such as Moscow might erupt.

“The fundamental difference between crisis management and collective defense is this: It is not we but our adversary who determines the timeline,” said Admiral Rob Bauer, one of NATO’s top military officials. “We have to prepare for the fact that conflict can present itself at any time.”

By outlining what it calls its regional plans, NATO will also give nations guidance on how to upgrade their forces and logistics.

“Allies will know exactly what forces and capabilities are needed, including where, what and how to deploy,” said NATO chief Jens Stoltenberg about the highly classified documents that will, as in the Cold War, assign certain troops to the defence of certain regions.

This formalizes a process triggered by Russia’s annexation of Crimea in 2014, which prompted Western allies for the first time to deploy combat troops to the east, with Britain, Canada and Germany each taking the lead in one of the Baltic states.