Nearly a week after the Ukrainian counteroffensive began in earnest, the Russian defenses are largely holding, and it’s clear that the Ukrainian military cannot count on another rapid, sweeping success akin to last fall’s Kharkiv region offensive, which saw up to 12,000 square kilometers of territory liberated.

This doesn’t mean the counteroffensive is failing or even faltering: Ukrainians have made clear tactical gains on at least one of the attack axes, and they may be helped in the future by the collapse of the Kakhovka hydroelectric plant’s dam as the Dnipro becomes shallower and easier to cross upstream from the flood zone. The lack of unity among Russian commanders is another potential success factor.

But now is still a good time to recall the assertion of some military theorists that modern technology has shifted the offense-defense balance in favor of defense. This school of thought provides a ready explanation for Ukraine’s surprising success at repelling the Russian invasion, but also suggests that the much-ridiculed Russian troops may have a natural advantage as they face the current Ukrainian onslaught.

As Seth Jones, Alexander Palmer and Joseph Bermudez point out in a recent brief for the Washington-based Center for Strategic and International Studies, technological developments that increased mobility historically favored the attacker and those that boosted firepower helped the defender.

If the introduction of the stirrup in the 8th century didn’t quite bring about feudalism in Europe, as U.S. historian Lynn Townsend White suggested in the 1960s, it did enable crushing cavalry charges that subsequently won many a battle. The tank enabled the most spectacular offensives of World War II. By contrast, the machine gun, the anti-aircraft gun and infantry anti-tank weapons all made it easier to repel attacks.

Most recently, defensive technology has arguably made the greater advances. U.S. military theorist Amos Fox has argued convincingly that maneuver is dead as a military strategy because modern war is increasingly fought in cities and the modern battlefield is saturated with surveillance technology that reduces mobility and the element of surprise. With drones hovering everywhere, it’s hard to move in open terrain without quickly attracting artillery fire.

These factors have contributed to Ukraine’s successful defensive campaign. So did the abundance of infantry antitank devices such as the U.S.-made Javelin and the U.K.-produced NLAW, as well as the use of long-range artillery (especially U.S.-made HIMARS with guided missiles) and relatively strong air defenses. Some of these components of success have been relatively cheap: A tank, even a Soviet-made one, costs much more than a Javelin or an entire swarm of commercially available drones.

The relative Russian helplessness in the face of modern defensive technology robbed the invading army of its expected freedom of maneuver, made it appear ponderous where speed was of the essence and thus enabled the Kharkiv breakthrough. Confused and demoralized by the formidable resistance it met, the Russian military had failed to pay proper attention to its own defenses.

Frank Hoffman of the Institute for National Strategic Studies at the National Defense University has called on the U.S. and NATO to adapt their strategies to these changing technological conditions:

The massive employment of guided weapons, including rocket and missile artillery, in Ukraine is instructive. The combination of pervasive surveillance and deeper strike systems afford a defensive but operationally relevant advantage that NATO should base its defensive strategy around.

As a direct consequence of the Ukrainian success at defense, however, Russia itself has been put on the defensive as it tries to hold on to captured territory. Now, the advantages afforded by surveillance technology and long-range artillery play into its hands. Russian troops in the field often have been as inventive as their Ukrainian adversaries in putting to use makeshift drone technology and the Russian artillery is at least no weaker than the opposing side’s. Gen, Sergei Surovikin, who, in his brief time as the Russian military operation’s supreme commander, surrendered the city of Kherson because he correctly assessed it as indefensible, proceeded to spend months planning and constructing several lines of defense, sometimes scaring Russian soldiers by how far into the rear the fortifications were being erected.

A defending Russian army also brings to bear air defenses that have adapted to Ukraine’s use of Western-made guided missiles and a formidable electronic warfare capability. According to a recent report from the U.K. Royal United Services Institute,

Russian electronic warfare remains potent, with an approximate distribution of at least one major system covering each 10 km of front. These systems are heavily weighted towards the defeat of UAVs and tend not to try and deconflict their effects. Ukrainian UAV losses remain at approximately 10,000 per month. Russian EW is also apparently achieving real time interception and decryption of Ukrainian Motorola 256-bit encrypted tactical communications systems, which are widely employed by the Armed Forces of Ukraine.

In a way, both the Russian and Ukrainian militaries are more in their element when defending rather than attacking. The first week of the long-delayed Ukrainian assault has shown that the Ukrainian military failed to exploit an element of surprise anywhere it tried to apply pressure. And while Russian soldiers were apprehensive about the new Western tanks and other armored vehicles newly supplied to Ukraine, the first losses of German-made Leopards and U.S.-made Bradleys have shown these are hardly indestructible.

Demoralized or not, the regular Russian military didn’t run at the sight of the advancing Ukrainians, who had trouble breaking through even the first defensive line. The one significant exception so far is the erstwhile Vremivka Salient that jutted out into Ukrainian-held territory in the southwest of the Donetsk region. According to official Ukrainians reports backed up by Russian Telegram feeds, it has been all but erased as Ukrainians retook several villages, pushing the invaders to their second line of defense. Ukrainian forces also have advanced slightly in the vicinity of the town of Bakhmut, only recently claimed by the Russian forces.

The Ukrainian military’s only hope of acquiring an element of surprise lies in the rapid terrain changes following the destruction of the Kakhovka dam. The Russian defensive plans do not appear to have incorporated such a scenario.

The Russians’ job is to insure themselves against any such surprises and to plug any emerging holes in their defenses by bringing in reserves. The Russian command appears to be relatively comfortable with this mode of operation — otherwise Defense Minister Sergei Shoigu wouldn’t have made a major move against the many irregular and semiregular units that have fought alongside the Russian military since the invasion began. He has ordered all members of these units to sign contracts with the ministry, decreeing the kind of transformation that Ukraine itself has undergone since the first battles against Russia in 2014: Its volunteer battalions have been fully integrated into the regular military.

Yevgeny Prigozhin, founder of the Wagner mercenary army, responded defiantly, saying his troops would not sign any contracts "with Shoigu.” But then Wagner, exhausted and decimated by a 220-day battle for Bakhmut, doesn’t have a role in the defensive campaign, at least not yet. That gives Shoigu, who has long sought a way to neutralize Prigozhin, confidence to try to sideline him in a minicounteroffensive of his own. But to be able to consolidate control, the minister needs to thwart the Ukrainian onslaught while relying almost exclusively on regular and obedient forces. In a way, though, these Byzantine intrigues within the Russian command represent Ukraine’s best hope: When the enemy force is not united by a common purpose, it cannot be uniformly strong no matter how it tries.

In their brief, Jones, Palmer and Bermudez hold up the example of Heinz Guderian’s XIX Panzer Corps, which broke through at a single point of France’s supposedly formidable Maginot Line in May 1940, and thus rendered the entire line useless. The Ukrainians appear to be looking for a weak spot to exploit in a similar way — a fortification badly built by a corrupt contractor, a poorly trained or poorly led Russian unit, a breach of discipline caused by commanders’ bickering — to prove that maneuver isn’t, in fact, dead.

Leonid Bershidsky, formerly Bloomberg Opinion’s Europe columnist, is a member of the Bloomberg News Automation Team. He recently published Russian translations of George Orwell’s "1984” and Franz Kafka’s "The Trial.”