With each day that Ukrainian forces remain inside Russia the risk increases that they could, conceivably, find themselves overwhelmed by superior force of numbers.
"The Russian army tends to move slowly," says a senior British defence source, "but once it cranks into gear it presents a sizable force."
Ukraine is critically short of manpower in this war and it certainly can't afford to lose some of its best remaining combat troops who have carried out this incursion so skilfully.
"The moment the Russians start redeploying some of their best troops in size to tackle this incursion, "says another senior British defence source, "that's the moment the Ukrainians need to withdraw back across the border."
Military experts reckon that after some early successes, Ukraine has made a number of costly strategic mistakes in the war.
Sacrificing so many of its best troops in the ultimately failed defence of Bakhmut and Avdievka in eastern Ukraine has been a terrible blow.
Some go further, accusing President Zelensky of a disastrous error of judgement in not mobilising more of his country’s young men from the start. "This is a war of survival for Ukraine," said one.
"Delaying conscription is not a luxury it has."
