The stark warning by General Robert Neller came as Boris Johnson became the first foreign secretary for six years to visit Moscow, with both sides conceding relations are at a “low point”.
Addressing 300 US Marines on a Christmas visit to their base in Norway, Gen Neller, who heads the 200,000-strong US Marine Corps, told his soldiers to be prepared for a “big-ass fight”.
He said: “I hope I’m wrong, but there’s a war coming.
“You’re in a fight here, an informational fight, a political fight, by your presence.”
He later told the men of 2nd Battalion, 2nd Marines, that the US’s next “operational points of focus” would be Russia, citing conflicts with Ukraine and Georgia, and the Pacific, as Washington slowly disentangles itself from 20 years of Middle Eastern involvement.
Russia has warned neighbouring Norway that its decision to accommodate a new unit of US Marines to the end of 2018 could hurt relations.
Britain deploys more than 7,200 troops in Estonia, Latvia, Lithuania and Poland as part of a Nato effort to deter Russian aggression in eastern Europe, after its 2014 intervention in Ukraine.
Marine Corps Sergeant Major Ronald Green added: “Just remember why you’re here.
“They’re watching.
“Just like you watch them, they watch you.
“We’ve got 300 Marines up here.
“We could go from 300 to 3,000 overnight.
“We could raise the bar.”
Mr Johnson maintained that key issues, such as Russia’s human rights record, its ambitions in the Ukraine, in the western Balkans and its cyber attempts to influence elections could not be ignored, adding that the UK was “willing and able” to respond to cyber attacks in kind.
Experts warned last night that any attempts at genuine rapprochement would have to wait until Vladimir Putin was no longer in power.
The Russian president recently announced he would stand again for elections next year and is widely expected to win, meaning he will continue until at least 2022.
Dr Andrew Foxall, of the Henry Jackson Society think-tank, said: “UK-Russia relations are in a difficult place.
“The problem is that, while Russia recognises that Britain is Europe’s nuclear guarantor and Nato’s driving force here, it sees itself as a great power and doesn’t view the UK as an equal.
“On that basis, I really don’t think Boris Johnson’s visit will achieve anything substantial, because nothing will change while Putin remains in power.”
He said the foreign secretary’s warning about cyber strikes against Russia showed the true nature of the relationship.
Dr Foxall added: “Britain has long held both an offensive and defensive cyber capability.
“This sets us apart from many other countries.
“The point of Boris Johnson making this warning public was to create a trip wire – to inform the Kremlin that there will be consequences to its actions.
“And that’s where we’re really at.”