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'.
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