CNN — Ukraine says its preparations for a spring counter-offensive are almost complete. When it’s launched, probably in the south, it will mark a pivotal moment in the conflict. But the Russians have had nearly six months to prepare the ground – and build an elaborate array of defenses. Breaking through will present a huge challenge.

Satellite imagery reviewed by CNN and other news organizations shows the extent of Russian defenses that have been built up in parts of southern Ukraine – layers of anti-tank ditches, obstacles, minefields and trenches.

The defenses continue for hundreds of miles across the meandering southern front – where Ukrainian forces are expected to concentrate their counter-offensive in the coming weeks.

The challenge for the Ukrainians will be to bypass or overcome such obstacles at speed, creating momentum that causes Russian command and control to melt down.

Several satellite images shared with CNN by Maxar Technologies, and taken on Wednesday, show extensive trenches east of the town of Polohy in Zaporizhzhia region. A Reuters’ analysis of imagery found thousands of defensive positions over a vast area.

They showed that “Russia’s positions are most concentrated near the front lines in the south-eastern Zaporizhzhia region, in the east and across the narrow strip of land connecting the Crimean Peninsula to the rest of Ukraine,” according to the Reuters analysis.

Russian defenses included, for example, anti-tank ditches near Polohy stretching for 30 kilometers (19 miles), as well as extra fortifications around important towns like Tokmak. This area will be critical should Ukrainian forces try to advance towards the city of Melitopol and split Russian forces in the south.

Maxar’s Stephen Wood says these defenses are replicated across a huge stretch of territory, from Crimea in the south all the way to parts of Donetsk.

CNN has previously reported on defensive fortifications being built in north Crimea.

The Russian-appointed head of Crimea, Sergey Aksyonov, said earlier this month that “our armed forces have built a modern, deeply-echeloned defense.”

Months of preparation

These defenses began to appear after Russian forces withdrew from part of Kherson region in November, and essentially established a new defensive line stretching across largely rural areas of southern Ukraine. The UK Defense Ministry said in November that two factories were producing concrete “dragons’ teeth” tank obstacles.

Such defenses, however, are only as good as the forces assigned to each sector. On their own they are a limited impediment. Which is why the Russians have pushed more units into southern Ukraine. These defensive lines have become critical to their overall goals.

Ukrainian officials have frequently relayed accounts from people in occupied areas such as Mariupol and Berdyansk of long Russian convoys passing through and dozens of buildings being appropriated as military accommodation.

Satellite imagery shows that a large Russian base in northern Crimea that in February had been full of equipment, including artillery and tanks was much emptier in late March and almost completely vacated by last week. It’s unclear where the equipment went but likely that it was sent north to reinforce Russian defensive lines.

Even so, it’s exceptionally difficult to tell how many Russian troops – and of what quality – are assigned to each section of such a long frontline. It will be critical for the Ukrainians to disrupt supply lines, destroy ammunition depots and hit fuel infrastructure (among many other tasks) ahead of the offensive.

That will make sustaining those defending troops harder.

The Ukrainians will be assessing where Russian weaknesses are because momentum once the counteroffensive begins will be critical.

No surprises

Ukrainian officials have acknowledged that unlike last September’s sudden sweep through much of Kharkiv region, they may lack the element of surprise in any larger counter-offensive.

Russian-appointed officials in Zaporizhzhia claim there is already a large build-up of Ukrainian forces in the area. Vladimir Rogov, head of “We Together with Russia” in Zaporizhzhia said Thursday that several new Ukrainian brigades were due to arrive at the front line by the end of this week.

“These brigades are being transferred to the existing 12,000 fighters in the area,” Rogov said. His claim could not be verified.

Ukrainian officials do not disclose the movement of units.

NATO officials say that 98% of the fighting vehicles pledged to Ukraine are now in-country and the Ukrainian Defense Mnister, Oleksii Reznikov, said Friday that preparations for the counter-offensive are almost complete.

But Ukrainian units will have to master combined arms maneuvers with this new equipment, integrating mine-clearing, the removal of tank obstacles and bridge-building with their assault battalions. That is complex coordination.

The package of US aid announced in March included armored vehicle-launched bridges, which would accompany advancing units — as well as demolition munitions.

And they will have to work with excellent coordination and communication to succeed. Some analysts have compared what the Ukrainians need to do with the D-Day landings, of which German General Erwin Rommel said at the time: “The first twenty-four hours of the invasion will be decisive…for the Allies, as well as Germany, it will be the longest day.”

Franz-Stefan Gady, a London-based specialist in modern warfare, says the Ukrainian goal must be to “set off paralysis in the Russian military leadership and panic across the Russian rank and file … Intangible factors such as tactical surprise, battlefield leadership, and fighting morale will likely be decisive in the first 24 hours of an attack.”

In an ideal scenario, he says, “Ukrainian armored columns punch through layered Russian defenses at a weak spot, quickly advance into the Russian rear, and threaten command and control nodes like military headquarters and supply centers.”

But the danger for Ukrainian forces, according to Matthew Schmidt, associate professor of national security at the University of New Haven, is that the counteroffensive becomes “a division level fight devolving into a series of platoon engagements,” with the Ukrainians getting bogged down.

Mastering combined arms – the use of a variety of assets in coordination – will be crucial, he says. “Attack supply depots in the rear, clear mines fast, coordinate fires and movement from brigade to platoon level.”

The Ukrainians do have the advantage of choosing where and when to go, and with what concentration of forces. Rogov, the Russian-appointed official in Zaporizhzhia, says he expects the Ukrainians to launch several diversionary attacks to try to confuse Russian defenses, especially with the use of small reconnaissance groups across the river Dnipro in both Zaporizhzhia and Kherson.

Once the assault begins, other factors could come into play: everything from the weather to the Russians’ capacity and desire to counter-attack, and the aerial component.

A Russian counterattack?

One attribute of successful defense is the ability to counterattack, to throw the advancing enemy off balance and force it to send troops where it would rather not. The Russians’ ability to do this effectively is in doubt. Western analysts believe elite forces such as Russia’s VNV (or VDV?) paratroopers took heavy losses early in the campaign, from which they are yet to recover.

“Despite the tactical adaptations we have seen from the Russians, we are yet to see them achieve an operational breakthrough and exploitation during the 2023 offensives,” says Mick Ryan, a former Australian general who writes the military strategy newsletter Futura Doctrina, and was recently in Ukraine.

But the Russians retain a distinct advantage in the air, and this could be critical in slowing Ukrainian progress. Ryan notes that to avoid Ukrainian air defenses, the Russian air force have increasingly used ‘stand-off’ weapons such as the 1.5 tonne glide bombs that have recently been used around Bakhmut.

“Not only do these increase the survivability of the launch aircraft, but they are also very difficult to intercept,” says Ryan.

Despite the greater ingenuity and motivation of the Ukrainians, and despite the Russians’ staggering losses since the invasion began, Moscow retains far superior resources in this conflict.

Gady tweets: “In the long run, Ukraine’s armed forces will have a tough time escaping the crucible of attrition of this artillery-focused land war.”

And even if they succeed in breaking through Russian lines, and reaching Melitopol and Berdyansk, there’s little expectation among Western officials that Russian President Vladimir Putin will change his objectives in Ukraine.

Schmidt makes the point that military force only works when it induces a political effect. “That means Putin needs such a major loss he can’t deflect it…and that would be taking Crimea.”