The Armed Forces must be prepared to "fight and win" to prevent the spread of war in Europe, the new head of the Army warned.
Comparing the situation to the build-up to the Second World War in 1937, the Chief of the General Staff said Britain must be prepared to "act rapidly" to ensure it is not drawn into a full-scale conflict through its failure to contain Russian expansionism.
In a speech on Tuesday, General Sir Patrick Sanders said he had never seen such a clear threat to the principles of sovereignty and democracy as the "brutal aggression" of Russian president Vladimir Putin.
His latest warning, in an address to a conference organised by the Royal United Services Institute (RUSI) defence and security think tank, comes after he wrote to all troops under his command telling them they must prepare "to fight in Europe".
In his first public engagement since taking up his post, Gen Sanders said his focus is on mobilising the Army to help prevent the spread of war in Europe by being "ready to fight and win alongside our NATO allies and partners".
Gen Sanders also Britain and NATO must "meet strength with strength from the outset and be unequivocally prepared to fight for NATO territory".
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"This is our 1937 moment. We are not at war – but we must act rapidly so that we aren't drawn into one through a failure to contain territorial expansion.
"I will do everything in my power to ensure that the British Army plays its part in averting war."
Gen Sanders also said: "We must, therefore, meet strength with strength from the outset and be unequivocally prepared to fight for NATO territory."
Despite the recent emphasis on new capabilities, such as drones and cyber warfare, he said that land forces will remain crucial in any conflict, adding: "You can't cyber your way across a river."
In a reference to the start of the First War World, Gen Sanders will say that "this is not the rush to war at the speed of the railway timetables of 1914".
Watch: Meet the new British Army chief.
Instead, the Army chief said this will be an "acceleration of the most important parts of Future Soldier's bold modernisation agenda" and named the process Operation MOBILISE, which will be "the Army's primary focus over the coming years".
Gen Sanders also warned any further cuts to the size of the British Army would be "perverse".
He said: "Obviously our Army has to be affordable; nonetheless, it would be perverse if the CGS was advocating reducing the size of the Army as a land war rages in Europe and Putin’s territorial ambitions extend into the rest of the decade, and beyond Ukraine."
Speaking at the same event, Defence Secretary Ben Wallace issued a fresh call for increased defence spending in the years ahead to counter the growing threat.
It comes as Prime Minister Boris Johnson prepares to join other NATO leaders in Madrid for a summit where they are due to agree the biggest overhaul of the Western military alliance since the end of the Cold War.
On Monday, NATO Secretary General Jens Stoltenberg announced the alliance is set to increase the numbers of its rapid reaction force – boosting its numbers from the current 40,000 to more than 300,000.