Britain and its allies face their "1937 moment" following Russia's invasion of Ukraine and must do everything possible to avert another world war, the new head of the Army is set to say.
The warning emerged as Prime Minister Boris Johnson is expected to announce an uplift in UK defence spending this week, in line with growing security challenges.
General Sir Patrick Sanders, the Chief of the General Staff, is set to say Russian President Vladimir Putin and his "expansionist ambitions" pose the greatest threat to sovereignty, democracy and the freedom to live without violence that he has ever known.
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His comments, at an annual army conference on Tuesday in London, will be made as Mr Johnson and fellow leaders of the 30-member NATO alliance prepare to meet later in the day in Madrid for a landmark summit, dominated by the West's response to Russia's war.
General Sanders, who took over as chief of the army last month, is expected to say his sole focus is "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".
His warnings will be echoed by Defence Secretary Ben Wallace, who is also expected to speak at the army conference before he travels to attend the NATO summit.
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The defence secretary is set to indicate a desire for greater spending on the UK's armed forces from 2025, following what has already been a multi-billion-pound boost to defence spending this parliament.
"The defence secretary is expected to emphasise that now that the threat has changed, governments must be prepared to invest to keep us safe," a defence source said.
Another defence source said: "We do not comment on alleged leaks. The defence secretary and the prime minister have always said that the government will respond to any changes in threat, which is why in 2020 the Ministry of Defence received a record defence settlement."
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With Russia's invasion changing the security landscape in Europe, the head of the NATO alliance has already revealed that allies will significantly increase the size of a high readiness force to more than 300,000 personnel up from around 40,000.
General Sanders is expected to offer his view on the threat posed by Russia and how his army is adapting, with a greater focus on urban combat and rebuilding costly stockpiles of weapons – allowed to be hollowed out to save money since the end of the Cold War.
It is a move that will require all ranks from general to lance corporal to "get ready, train hard and engage", he is expected to say.
"This is our 1937 moment," the army chief will say, according to excerpts of the speech that were released in advance. He is referring to the crucial period leading up to World War Two.
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"We are not at war – but 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."
The challenge means the army must modernise, embracing new technologies such as cyber warfare and long-range missiles, but also retain traditional soldiering skills.
General Sanders is expected to say if a battle came "standoff air, maritime or cyber fires are unlikely to dominate on their own – land will still be the decisive to domain", adding that "you can't cyber your way across a river."
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He is set to say that the army's mobilisation is "not the rush to war at the speed of the railway timetables of 1914" but is an "acceleration of the most important parts of Future Soldier's bold modernisation agenda… an increased focus on readiness and combined arms training."
Future Soldier is the name given to the army's plans for its capabilities.
This will mean more training on combining the different domains of warfare – land, sea, air, cyber and space as well as rebuilding stockpiles.
General Sanders is also expected to say the army will "review the deployability of our vehicle fleet".
This could be a signal of a decision to be made about a multi-billion-pound programme to develop a mini-tank called Ajax that has been beset with problems and has yet delivered.
The top officer is expected to say deterring Russia "means more of the army ready more of the time" and that he expects "all ranks to get ready, train hard and engage."