The new army chief said British troops must prepare “to fight again in Europe” as the conflict in Ukraine continues.
General Sir Patrick Sanders took over from General Sir Mark Carleton Smith on Monday and wrote to his charges about the challenges they face.
“There is now a burning imperative to forge an army capable of fighting alongside our allies and defeating Russia in battle,” Sir Patrick said.
“We are the generation that must prepare the army to fight again in Europe,” he added.
It comes after the Prime Minister of Ukraine returned with a warning that “we must arm ourselves for a long war”.
After his visit to Kyiv, Boris Johnson said Vladimir Putin’s invasion was “entering a new phase” and that if Russian advances were successful, it “would not stop at the dismemberment of Ukraine”.
Writing in The Times, Mr Johnson said: “I fear we need to arm ourselves for a long war as Putin resorts to a campaign of attrition, trying to crush Ukraine through sheer brutality.
“The UK and our friends must respond by ensuring Ukraine has the strategic stamina to survive and ultimately prevail.”
“Everything will depend on Ukraine’s ability to strengthen its ability to defend its soil faster than Russia can renew its ability to attack,” he added.
“Our task is to buy time on the side of Ukraine.”
The government presented plans last year to reduce the size of the regular army, lowering the target size from 82,000 personnel to 72,500 by 2025. When the announcement was made in March 2021, the army numbered approximately 76,500 regular soldiers.
Sir Patrick said he was “the first Chief of the General Staff since 1941 to take command of the army in the shadow of a ground war in Europe involving a continental power”.
He added: “Russia’s invasion of Ukraine underscores our primary objective of protecting the UK by being prepared to fight and win wars on land.”