Russia could overrun Nato's current military force in the Baltic states in a matter of hours, according to a US think-tank.
In a stark report, the US Army-linked Rand Corporation said Nato's military assets in Estonia, Latvia and Lithuania were so mismatched with Russia as to be "inviting a devastating war, rather than deterring it".
The think-tank said it carried out a series of war games exploring a scenario in which Russia tried to annex the capitals of either Estonia, Latvia, or both, similarly to how it seized the peninsula of Crimea in 2014.
"The outcome was, bluntly, a disaster for Nato," the report found, with every playthrough of events ending with Russian forces in or at the gates of Tallinn and Riga within 60 hours.
With Russia outnumbering Nato ground battalions by almost two to one and possessing much heavier weaponry than the Baltic defence forces, some simulations had the capitals falling in as little as 36 hours from the start of hostilities, the Independent reported.
"As presently postured, Nato cannot successfully defend the territory of its most exposed members," the think-tank concluded.
In recent weeks alone, Russia has moved battleships toward the Mediterranean and Baltic Seas, shifted nuclear-capable missile-launchers into its Kaliningrad enclave neighbouring Poland and continued flying bombers down the western European coast.
And in a separate report, Rand observed that Russia was more than capable of "ignoring norms on territorial aggression when it sees fit".
As part of its simulations, Rand imagined what action would be available to Nato if Russia chose to take Tallinn and Riga, describing them as "a limited number of options, all bad".
It said the alliance would have to choose between three options: to launch a major counter-offencive and take back the cities in a prolonged conflict; to threaten Moscow directly in a bid to force Russia to withdraw; or to accept the military development, at least in the short to medium term.
The first two options, Rand suggested, could quickly escalate to the potential use of nuclear weapons. The latter would, at best, lead to "a new cold war".
Speaking on Thursday, the alliance's top military commander, US General Curtis Scaparrotti, said: "What we are observing is Russian military practice that diverges widely from NATO practice in scale, scope, content, purpose, and transparency.
"We need to be strategic and coherent in our approach to defending our citizens. Now is the time for the Alliance to remain strong."