https://www.lbc.co.uk/news/russia-lose-million-soldiers-to-achieve-ukraine-war-goals/
General Sir Roland Walker said Moscow would take another five years to attain their minimum objectives in Ukraine.
The numbers of dead and wounded would top 1.5 million, he said in a speech at the Royal United Services Institute conference in London on Wednesday.
Russia has already lost 550,000 men, either killed, wounded or captured since it invaded, over two and a half years ago.
Sir Roland said that a long war produces "no winners," adding that "it is an utter devastation for both sides and lost generations."
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He said the war in Ukraine was "not going well for anyone", adding that success for Moscow would threaten the NATO security alliance.
Sir Roland added: "It has just started in Ukraine, so it must fail in Ukraine, otherwise where does it go next".
If they carry on as they are, it would probably take the Russians five years to grind their way to their minimum objectives of the four oblasts."
This was a reference to the four regions of Donetsk, Luhansk, Kherson and Zaporizhzhia that Russia claims as its own, and which are under partial Russian control.
Sir Roland said that taking those regions entirely would cause Russian casualties to skyrocket.
He said: "At the current rate of attrition of dead and wounded, that puts them probably well north of 1.5 million people casualties to achieve that, with untold billions of lost equipment.
"There have got to be more things for Russia to worry about than losing the best part of 1.5 to 1.8 million people for a slice of Ukraine, with the way the world is going."
Ukraine has suggested that it needs to strike into Russian territory in order to turn the tide of the war.
Last week, Keir Starmer rebuffed a request from Volodymyr Zelenskyy to fire British missiles deep into Russia. Currently, Ukrainian forces can only fire the missiles in a defensive sense, at targets on Russian soil close to the border.
Sir Roland said he feared that Russia would seek retribution on the many Western countries that have banded together to support Ukraine since the invasion.
He said: "The lesson from history is the Russians don’t forget and they will come back to get their revenge."
It comes after Sir Roland warned in a speech on Tuesday that the UK and its allies are facing a growing threat from an ‘axis of upheaval’ consisting of Russia, China, Iran and North Korea, adding that “nothing about the end of the war in Ukraine will change that”.
He said: “If we can double, then triple, our fighting power, any British land force will be able to destroy a force at least three times its size and keep on doing that.
“That way we will have every confidence in being ready and able to fight anyone and win and it’ll also mean our soldiers have every reason to stay in the Army.
“We can double in three years with a hybrid system, and then triple when the new fighting systems come online.
“We will sense twice as far, decide in half the time, and deliver effects over double the distance with half as many munitions.
“Our Ukrainian partners are beginning to do this with great results now.”
