The Chief of the Defence Staff says Russia has been "flexing their muscles" in the UK’s backyard in a manner not seen since the Cold War.
In his annual Royal United Services Institute Christmas lecture, delivered online this year, General Sir Nick Carter warned Moscow wanted to test Britain and NATO allies.
The head of the UK military said: "They are wrestling with their own sense of imperial overstretch as their ‘near abroad’ becomes increasingly restive.
"The week before last, Russia assembled 10 or so warships and combat aircraft from the northern Baltic and Black Sea fleets in a show of force in the waters off the British and Irish coasts.
"They are flexing their muscles in our own backyard with an ostentation that they’ve not displayed since the Cold War.
"Deterring these threats, signalling to the Russian regime that we shall not tamely acquiesce should they escalate, requires conventional hard power, warships and aircraft, as well as less conventional capabilities like cyber."
Gen Sir Nick outlined the importance of maintaining strong relations with allies such as NATO.
He also warned that the UK’s "authoritarian rivals" are gaining the edge in strategic warfare.
The CDS said the UK's campaigns over the last 30 years have been closely watched by rivals, who are attaining a technological advantage.
He also mentioned how the Prime Minister's announcement of a defence spending boost worth an extra £16.5bn over the next four years "reverses... a long period of decline".
Also included in his address, was the importance of cyber warfare in modern combat.
But the general, who has served in Bosnia, Iraq and Afghanistan, added: "Throughout, we must recognise that the nature of war doesn’t change. It’s always about violence, guts, people.
"When you are up against a determined opponent on the battlefield, you have to go close and personal with your enemy.
"I am afraid it’s too early to plot the demise of the tank."
The 61-year-old, who joined the Army in 1977, said the world was changing more quickly than ever before, but he was convinced the forces could adapt.
"We shall surprise and perhaps dismay some people who expect us now, as sometimes in the past, to be preparing to fight the last war," he said.
"Our business is instead with the next one and with arming, training and equipping ourselves to fight, if we must, but better still to convince our prospective adversaries that the game is simply not worth the candle."