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“The Kremlin has so far failed to achieve its original objectives. It has been surprised by the scale and ferocity of Ukrainian resistance,” the UK’s Ministry of Defence said on Saturday.
“Russia has been forced to change its operational approach and is now pursuing a strategy of attrition," the ministry said. A war of attrition means the outcome of the conflict depends on which side can sustain manpower, despite high losses, as both sides wear each other down.
With the exception of the south of the country, where Russia is making gains in the port city of Mariupol and has taken control of the cities of Kherson and Melitopol, Russia's advances have stalled on most fronts.
Russia is poised to occupy most of Ukraine’s coastline, with the large port city of Odesa being one notable obstacle. But Ukraine has also launched a number of successful counterattacks, notably near the city of Mykolaiv.
A current threat for Ukraine is the risk that Russian forces could cut off large numbers of Ukrainian soldiers, trapping them against the Dnieper river, which bisects the country.
"Ukrainian forces opposite Donetsk and Luhansk are at risk of encirclement on the eastern side of the Dnieper," said Sam Cranny-Evans and Sidharth Kaushal, writing on the UK's Royal United Services Institute website.
Whether or not Russian forces achieve this encirclement, losses on both sides are extremely high and mounting.
Recent US estimates put Russian losses between 2,000 and 7,000 while President Volodymyr Zelenskyy said Ukraine had suffered 1,300 dead soldiers by March 12.
The high losses on many fronts raise the question of whether Russia has aimed for too many objectives, rather than aiming to capture Kyiv with one major thrust.
"Napoleon told one of his more sluggish generals, 'if you're going to take Vienna, take Vienna,' Lt Gen Michael Barbero told The National.
"Because if you're gonna go for it, you better have everything, your logistics, your fires, the simultaneity of effects, you better have that all synchronised and lined up," he said, noting that Russia's considerable force might be too dispersed across the country.
Russian forces are trying to seize a number of large cities including Kyiv, which has a population of more than three million people, Kharkiv, which had a prewar population of 1.5 million and Odesa, a large port city of nearly one million people.
Experts say that in order to take control of enemy-held terrain, attacking forces need at least three times the number of soldiers as the defender.
In urban operations, this ratio could be higher: an Iraqi force of about 100,000 backed by western air power and advisers took nine months to take control of Mosul, a city defended by as few as 12,000 ISIS fighters, by one estimate.
The force ratio problem is now a challenge for Russia, said Rob Lee, a senior fellow at the Foreign Policy Research Institute.
“It is becoming a numbers game for Russia. Partial encirclements of Chernihiv, Sumy, and Kharkiv all require Russian forces as does the fighting in Mariupol. Other forces have to be used to secure supply lines from ambushes and hold occupied cities,” he said on Saturday.
But even with new innovations such as using drones to act as “spotters” for artillery, it is generally thought to be easier to defend an area than to attack, and Ukraine also uses similar tactics.
Tooth to tail
In addition to the so-called “three to one rule," Russia's objectives are complicated by two factors, what analysts call the “tooth to tail ratio” of a force and the fact that Ukraine has mobilised its reserves since February 24, as well as recruiting large numbers of volunteers.
The tooth to tail ratio is the number of soldiers available for front line combat at any given time (“tooth”) versus the "tail," the number of soldiers working in support roles — supplying fuel, food and ammunition, as well as securing rear areas.
The more Russia advances, the more soldiers will be held back to guard supplies and bases.
UK defence attache Mick Smeath recently said that Ukrainian harassment of supply lines was “severely limiting Russia’s offensive potential.”
A study by the McKinsey consultancy on modern militaries, including France, the US, Spain, Japan, South Korea — and Russia, found that on average forces could sustain a combat commitment of 26 per cent of the force.
If this is correct, Russia can send about 40,000 men in combat of the 150,000 men now inside Ukraine. But those front-line forces are also taking extremely heavy casualties.
Meanwhile, Ukraine’s military has swelled its ranks to about 300,000, outnumbering the Russian invasion force.
Ukraine of course also has the “tooth to tail” ratio to consider, but defenders have shorter supply lines meaning their logistical requirements could be easier to sustain.
“Once they get into the cities, offensive operations in urban terrain are a meat grinder,” retired Lt Gen Michael Barbero said.
“The time the Ukrainians have had to prepare roadblocks, blowing up bridges, preparing kill zones to channel the Russians into, and ambushes, is going to turn it into a more casualty-producing situation. And when you're in urban conflict, the consumption of supplies goes up ten-fold,” he says.