When Ukraine's military paused to regroup towards the end of 2022, extensive Russian fortifications designed to slow any Ukrainian advances started to spring up along, behind and sometimes far removed from the front lines.

Satellite images of thousands of new defensive positions reviewed by Reuters show Russia has been digging in at key strategic points in readiness for an offensive by a Ukrainian military rearmed with state-of-the-art Western weapons.

Stretching from the Russian city of Voronok down through eastern Ukraine and southwest to the Crimean Peninsula, new trenches, anti-vehicle barriers and revetments for equipment and material have appeared, said Brady Africk, an open-source intelligence researcher and an analyst at the American Enterprise Institute.

Russia’s winter offensive made few gains. As the war carries over into a second spring and Ukraine prepares to renew its own attacks with an arsenal of new Western weapons and freshly trained troops, the Russian military most likely sees prepared defences as the best chance of staving off a decisive defeat, said Rob Lee, a senior fellow at the Foreign Policy Research Institute.

“After the Kharkiv offensive, Russia kind of realized that defeat was possible - they could lose territory. I think that was a realization that Ukraine can do offensive operations,” he said. The fortifications are “an acknowledgement of the risk that Ukraine could make another breakthrough”.

The trenches

The most common - and easiest to construct - type of defensive work is a trench. U.S. Army field manuals say such fighting positions should be dug roughly armpit-deep, be in non-obvious locations, ideally using natural cover for concealment, and include a front wall of sandbags, rocks, dirt or other protective material.

Trenches offer obvious protection from bullets and can help infantry survive artillery barrages, which have been a prominent feature of the war in Ukraine. Fragments from an 80mm mortar round, for instance, can be stopped with just 30 cm of dirt, the Army documents say, meaning that only an explosion overhead will have much effect on emplaced troops.

Russian forces have dug many such trenches along what they see as key roads and junctions, and outside strategic cities, Africk said. At least two locations, Tokmak and Bilmak in the Zaphorizhzhia region, are encircled by defensive works.

Most of the trenches are built in a zig-zag or angular pattern, which limits enemy sight lines and offers additional protection from shell fragments coming from the side. In some cases, the trenches are clearly visible in satellite images, are not well concealed and are built in open spaces.

Being dug in does not automatically mean the defender has an advantage, said Mick Ryan, a retired major general in the Australian Army and a combat engineer. Poorly planned defensive positions might even make them worse off, he said.

“A dumb adversary dug in versus a smart one that is conducting maneuvers is a different kettle of fish,” Ryan said. “It all depends on how well-sited these obstacles are. … Once you dig yourself in, it’s very easy to find you.”

Obstacles and traps

Barriers can range from ditches and barbed wire to hardened concrete obstacles. They are meant to restrict an adversary’s ability to maneuver, or to funnel their forces into areas that make them more vulnerable to attack. For trucks and other wheeled vehicles that mostly stick to roads, this is an easier task: Barbed wire, tree trunks, and even deep holes can slow or stop them.

Africk said his research showed Russian anti-vehicle ditches and barriers such as “dragon’s teeth” - concrete pyramid blocks set in staggered rows to block tanks - were common in Ukraine. Jack-shaped steel “hedgehogs” use the same sort of mechanics, wrecking tracks or tipping tanks into vulnerable positions.

Tracked vehicles such as tanks and armoured personnel carriers can plow over or through most obstacles, as well as travel off road, which adds to the difficulty of stopping them. Although barriers can be effective, another common means of stopping such vehicles is a mine. These weapons are cheap, easily hidden, and some can be scattered via artillery - meaning personnel do not have to approach the area to place them.

Many, such as the Soviet-era TM-62, which has been widely used in Ukraine, will not detonate without the pressure or magnetic presence of a heavy vehicle. Others are “off-road” mines, such as the PARM systems Germany has supplied to Ukraine. They are deployed some distance from the target area and are triggered remotely, for instance with a tripwire, firing a small penetrator. All are devastating, even against heavy armour. Around Vuhdelar, a city in southern Ukraine that Russia pushed hard to capture during the winter, mines destroyed dozens of Russian tanks, armored personnel carriers and mine-clearing vehicles, Lee said.

“If Russia employs mines effectively as well, it will be difficult for Ukraine,” he said.

Special equipment is needed to clear a path through a minefield, Ryan said. Plows and rollers can be attached to tanks, or other heavy vehicles can fire line charges - ropes of explosives - to blow up mines. That sort of equipment, and more, has been included in all the recent U.S. and European security assistance packages, he said.

The weather is another major factor. Mud - which appears seasonally in Ukraine during a period known locally as bezdorizhzhia that starts around March 1 - can make maneuvering off roads difficult or impossible even for tracked vehicles. During the mud season in 2022, many photos and videos emerged of Russian and Ukrainian vehicles that had been bogged down and abandoned.

Danger and messaging

The Ukrainian military’s counter-attacks in 2022 came largely against Russian forces that had not dug in, and were more widely dispersed than in 2023 because they were occupying more territory.

To some extent, looking at where Russian forces have built visible fortifications can help show what their commanders see as important, retired major general Ryan said.

“The Russian assessment appears to be that the Ukrainians’ most likely and most dangerous place to attack is in the south, particularly in Zaporizhzhia,” he said.

Breaking through

The sheer scale of the Russian defensive works is not, in itself, an obstacle, Ryan said. Ukraine’s military does not need to attack every kilometre in every theatre. Rather, commanders will weigh the strength of the defences, the importance of an area to their objective and the forces they can bring to bear against it, he said.

“Just because an area has heavy defences, it doesn’t mean you will choose not to fight through it,” he said. “Sometimes that is just what you have to do.”

Concentrating forces, a cornerstone of classic military doctrine, could offer Ukraine an advantage at a specific point on the front and allow them to push into Russian rear areas. That could unravel other defensive areas and create a wider breakthrough, said Lee at the Foreign Policy Research Institute.

Tanks and other armored vehicles must operate alongside engineers, artillery, and even aircraft to defeat layered defenses, he said - an approach called “combined arms” - and recent shipments of Western military gear would help with that.

Competent planning by Ukraine could prove even more valuable than better weapons, Ryan said.

“The most important assistance they have received is not so much the equipment, but… the training of battalion and brigade and higher-level staff in these very complex combined arms activities,” he said. “You can’t just get a pickup team and do this. It is the most complex ground operation you can conduct.”

The next move

How the front lines change in 2023 will come down to where Ukraine chooses to focus, what forces it uses and how well-prepared Russia is in those areas, Lee said.

Fortifications, if built and used properly, could make a major breakthrough difficult - but with Western provision of artillery shells most likely at its peak, Ukraine’s military may not get another chance, he said.

The fortifications, though, like other aspects of the war, will leave scars well after the last shell is fired, Africk said, telling of how another researcher found a mark he was having trouble interpreting on the terrain in a satellite photo of Ukraine.

Eventually, he realised he was looking at the remains of a trench from World War II.

“These things are going to be around for a long time,” Africk said.