Ukraine has 12 combat ready brigades of 40,000 troops ready to launch an offensive against extensive Russian defences of trenches, anti-tank obstacles and minefields up to 3km deep.
With huge losses from the year-long fighting, Russia has 200,000 troops spread out over the 1000km front line.
If Ukraine’s tanks manage to break out the military expectation will be that Moscow’s generals will direct their reserves to blunt the armoured thrust and avoid their rear echelon being overrun alongside a collapse in morale.
But western officials have indicated that the all-important reserve is extremely limited.
“A key question is whether Russia has its own credible reserves to deploy in counter-offensive,” an official told a media briefing. “While it looks unlikely I'd say it's a striking observation that Russia probably doesn't have a dedicated reserve force of divisional level or bigger.”
Asked by The National to explain the potential impact of a breakthrough, the official said a “key factor” would be the Russian capacity to mobilise a reserve.
“Then critically, the Russians' ability to cauterise that gap, to bring reserves up to them to deny that breakthrough, that is really important,” he replied.
A major battlefield loss in the coming weeks could also have a significant “cognitive effect” on the Kremlin, which could see a severe weakening of President Vladimir Putin’s authority, he said.
Russia defence capabilities have also been further undermined by the shooting down by US-supplied Patriot missiles of six of its supposedly invulnerable Kinzhal hypersonic missiles.
The impact of images of Ukrainians “cleaving through Russian lines for the Russian border” could be considerable.
“It will be the cognitive effects on those in the Kremlin. and the chance to show President Putin that he cannot simply win by default by waiting out behind defensive lines.”
He added: “The cognitive effect on the battlefield activity, I think in this case is arguably more important than how many square metres of territory is taken.”
Another problem for the Russians was that their soldiers fixed in place on the front line were poorly trained and lacked supplies.
Furthermore, the official said there has been “a real degradation in the quality of Russian junior officers and NCOs that have taken significant attrition”.
On Tuesday, Ukraine is understood to have shot down all six of the Kinzhal hypersonic missiles that were targeting Kyiv’s air defence systems.
The weapons allegedly travel at 12,000 kph and can carry nuclear warheads, making a significant contribution to Russia’s military modernisation, but they have now proven vulnerable.
“This is a weapon which President Putin seems to have earmarked for a major role in Russia's future national defence because of assumptions around its invulnerability on their side,” the official said.
“The Kinzhals are not succeeding and it was thought these were impossible to shoot down by any weapon system.”
Key factors in the coming offensive will be Ukraine’s air defences keeping the Russian air force at bay, and Ukraine's missile armoury, including the recent British-supplied Storm Shadow cruise weapons.
“Precision strike will be really critical in the counter-offensive when that finally launches,” the official said.