General Sir Nick Carter stressed that he did not think Russia wants to go to war in the “traditional “ sense.
But he warned: “Russia could initiate hostilities sooner than we expect, and a lot earlier than we would in similar circumstances.”
He said Russia is “the arch exponent” of “exploiting the seams between peace and war”with tactics such as “bribes, corrupt business practices, cyber attacks, assassination, fake news, propaganda and military intimidation”.
The threat is so great that he is considering a rethink of bringing all British troops back from Germany and instead keeping fuel, ammunition and tanks prepositioned at bases there.
And the Army is reviewing how to deploy to Europe on a hurry.
Sir Nick also warned that Britain is falling behind adversaries such as Russia
His speech to the Royal United Services Institute comes with Defence Secretary Gavin Williamson battling Treasury pressure for cuts.
Sir Nick showed a three-minute Russian video boasting about the Kremlin’s burgeoning Armed Forces and said: “To avoid being surprised they believe in preemption.”
And he said Russia aims to create mobile missile domes which rain missiles on their enemies.
With the centenary of the end of First World War coming in November, he said: “We should be careful of complacency. The parallels with 1914 are stark.
“Our generation has become used to wars of choice since the end of the Cold War but we may not have a choice about conflict with Russia.”
Russia, he said, “represents the most complex and capable state-based threat to our country since the end of the Cold War”.
Sir Nick added: “We have to be prepared to fight the war we might have to fight with a sporting chance we will avoid it.”