A snap exercise in Russia's eastern military district later the same year was meanwhile the largest since the fall of the Iron Curtain it involved 160,000 troops.

Russia could potentially seize territory in a Nato state using its rapidly assembled forces for example, the Russian-speaking enclave of Narva in Estonia before the alliance had time to swing into action, forcing leaders to either declare war or swallow their pride.

Such a course of action would raise the prospect of a "slide into strategic conflict", which, "however unlikely we see that as being now, represents an obvious existential threat to our whole being", Sir Adrian added, hinting at the potential for nuclear confrontation.

Read MoreRussia sanctions over Ukraine fighting seen as 'symbolic' by experts

The prospect of a brazen Russian attack is one of the key drivers behind Nato moves to speed up its ability to deploy sizeable military units in the event of a crisis. The centrepiece of the alliance's shift in policy following a summit in Wales in September is a "spearhead" brigade-sized rapid-reaction force capable of deploying within 48 hours.

Nato is preparing to deploy "force integration units" in each of its eastern European member states. They will act as eyes and ears on the ground as well as preparing the way for the rapid deployment of Nato forces should they be required by building links and logistical plans with local military units and commands.

Read the rest here:

Prepare for Russian blitzkrieg: General