The Russians could invade Poland 'overnight' and the US needs to do more to beef up Nato defences in the area and send more missiles to the region to deter Moscow.
A 25-page document by the US-based Atlantic Council think thank says Nato needs to do more to 'counter a resurgent Russia'.
The report says: 'Even if Moscow currently has no immediate intent to challenge Nato directly, this may unexpectedly change overnight and can be implemented with great speed, following already prepared plans. The capability to do so is, to a large extent, in place.'
It says the timing of a Russian invasion could not be predicted but it could come as a result of Nato being 'distracted by another crisis' or as a reaction to a 'misperception of Nato's activities'.
The report goes on to say: 'Russia rarely disguises its true intentions. On the contrary, it has proclaimed them very publicly on various occasions, but, in general, the West has chosen not to believe Russia's declarations and disregards its willingness to carry them out.'
The Atlantic Council claims Nato would be slow to respond to an invasion and Russia would use its nuclear weapons as a deterrent to prevent it turning into a full-scale war.
The report goes on to say Nato forces in Poland would be expected 'to delay and bog down an invading force and inflict unacceptable damage on it'.
It says: 'The [Nato] force [in Poland] is not required to win the war, but it must be able to fight alongside the host-nation forces to buy Nato more time for reinforcement. Nato's presence in the region is currently not large enough to achieve this.'
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