CNN tours one of the war fronts between Russia and Ukraine 2:53
Editor's Note:
Daniel L. Davis is a Senior Fellow and Military Expert at Defense Priorities and a former US Army Lieutenant Colonel who fought in the largest tank battle of the Gulf War, the Battle of 73 Easting.
He is the author of "The Eleventh Hour in 2020 America."
The opinions expressed in this comment belong solely to its author.
(CNN) --
Ukraine inflicted a stunning defeat in the face of Russian President Vladimir Putin's feeble attempt to capture the capital, Kyiv, in the first round of the war.
Now, Russia is preparing to launch a revised second phase in the east, shaping up to be the biggest tank battle in Europe since World War II: the Battle of Donbas.
After meeting fierce resistance north and west of Kyiv, and suffering appalling casualties, Russia's heavy tank force abandoned its hopes of capturing the city, and in late March began withdrawing its troops. .
What do the images of damaged Russian trucks say about the problems of its Armed Forces in Ukraine?
Putin's main goal, however, seems less the capture of geographic territory in Donbas than the destruction of the Ukrainian Armed Forces (UAF) defending in the east.
Militarily, it seems a much more achievable goal than Kyiv's initial goal.
Outside the urban capital, Ukrainian defenders will not have the protection of buildings to shelter them from Russian attack, and Putin's troops will be able to use their mobility to their advantage.
Before the outbreak of war on February 24, Ukraine had already positioned the majority of its army in the eastern Donbas region, focusing on the line of contact between Kyiv troops and Russian-backed separatists defending areas of Luhansk and Donetsk.
Reports estimate that there were up to 40,000 Ukrainian soldiers fighting on the Donbas front when the war broke out.
And there has already been fierce fighting there for the last six weeks.
But now it seems that both Putin and Ukraine's President Volodymyr Zelensky realize the importance of winning the "Battle of Donbas" in the great trajectory of the war.
If Zelensky can hold the line in Donbas, it will be virtually impossible for Putin to take the war further and buy Ukraine time to build an offensive force that could then drive Russia out of the country.
If Putin's forces are successful, they will capture or destroy a large percentage of all trained Ukrainian forces, and there will be little to prevent Russian armor from capturing Kharkiv, Odessa, or even returning to Kyiv.
Leaving widespread evidence of war crimes behind its withdrawal from Kyiv, Russia's repositioning in the east portends a potentially greater danger to Ukrainian troops.
Russia used a similar approach to capture the city of Mariupol.
They first surrounded the defenders to cut off any ability for Kyiv to get food, fuel, or ammunition to its troops.
The Russians then unleashed a torrent of heavy artillery on the parts of the city where the Ukrainian troops were holding out.
And finally, the Russians slowly closed the circle and captured or killed the defenders.
The Ukraine only gave up the fight when it ran out of water and ammunition.
Pro-Russian separatists in the Donetsk and Luhansk regions declared independence from Ukraine in 2014. Fighting between Russian-backed rebels and the Ukrainian army in this area has killed more than 14,000 people since April 2014.
Putin will try a similar strategy in Donbas.
And if Russia can outflank the Ukrainian forces in Donbas and cut off their supplies, she can, in time, come full circle.
Eventually, deprived of fuel, ammunition, food and water, the defenders, like those at Mariupol, will fall.
Up to this point, Russian and separatist troops have made progress against the Ukrainian forces defending Donbas.
That balance now risks tipping decisively toward Russia.
Approximately 70,000 Russian armored troops are moving into a position that will allow them to launch an offensive in the north of the Ukrainian defensive group opposite Donbas.
According to reports, Russia has also mobilized more than 60,000 new recruits and reservists to inject fresh troops into the fight.
Putin also brought in a notorious new Russian commander, General Alexandr Dvornikov, who led Putin's troops in Syria and became known as the "butcher of Syria."
To secure its own flanks, Russia is already carrying out a heavy bombardment of defending Ukrainian positions in Kharkiv and in the southwestern region of Kherson.
Russian Navy evacuates a flagship in the Black Sea before it sank.
Ukraine claims it attacked it with a missile
Both operations are designed to keep Ukrainian troops in place and prevent them from moving to strengthen Ukraine's defenses on the Donbas front.
However, Ukraine's difficulty in reinforcing and resupplying its Donbas force exposes a serious vulnerability for Zelensky's forces.
One of the things I learned from my tenure as second-in-command of the divisional armored cavalry squadron of the 1st Armored Division in Germany in the mid-2000s is that often the most important key to the outcome of a battle is something much less visual.
catch you rockets, missiles and battle tanks.
It's logistics.
Russia's inability to adequately resupply was a central failure of its ill-fated advance on Kyiv in February, and looms as a major factor in the impending Donbas fighting.
Preparing for the opening of the main striking force to the north and south of the Donbas battle group, Russia has been relentlessly attacking Ukrainian logistics lines from western Ukraine and the capital.
Russian fighter jets and bombers, attack helicopters, precision-guided missiles, hunting drones and long-range artillery have been inflicting heavy damage on Kyiv's resupply lines, stopping or delaying the three most crucial needs for a defense force: food, fuel and ammunition.
Without fuel, especially, Ukraine's troops will not be able to carry out the mobile counterattacks that are essential for defense forces in armored warfare.
They must be able to cut off enemy penetrations.
However, without sufficient fuel, the Ukrainians will be severely limited in their ability to cut off and repel Russian attacks.
Ukraine's best course of action is to continue to improve its existing defensive positions, stockpile as much food, fuel and ammunition as possible before the battle heats up, and do everything in its power to prevent any major penetration of its north or south groups.
If Ukraine can store enough supplies for its mobile tactical reserves and maintains the ability to reverse any Russian penetration, it will have a chance of slowing Russia's advance and avoiding encirclement.
That, too, will buy Ukraine's leaders time and give them clout to negotiate with Putin as casualties mount for Moscow.
However, if Russian armor manages to break through either battle group, it is possible that they could encircle the Ukrainian force in Donbas and methodically deprive them of critical resources.
In that case, the Ukrainian forces will slowly lose the ability to continue resisting, as they will run out of ammunition, food and fuel.
The stakes for each side in this impending battle could not be higher.
This is how Mariupol resists the advance of Russia 0:57
Russian invasion of Ukraine Vladimir Putin