According to BBC News, the head of the British Army, General Sir Nick Carter, has warned that Russian aggression could be a reality sooner than later and that the U.K. must prepare to "fight the war we might have to fight".
The exhortation comes at time there is speculation of defense cuts and after Russian attack simulations in Europe.
In the speech, at the Royal United Services Institute, Gen Carter highlighted Russia's new cyber warfare capabilities.
He also spoke about Russian long-range missile strike capability, which he said was demonstrated in Syria when 26 missiles were deployed from a 1,500km (930 mile) range.
Gen. Carter stressed that the U.K. and Russia are not on equal footing in terms of military might, describing Russia as having "eye-watering quantity of capability".
He said he did "not in any way" want to suggest that Russia would go to war in the traditional sense, but that Moscow "could initiate hostilities sooner than we expect".
"I don't think it will start with little green men, it will start with something we don't expect. We should not take what we have seen so far as a template for the future."