For over a decade, Russia spent hundreds of billions of dollars restructuring its military into a smaller, better equipped and more-professional force that could face off against the West.

Three weeks into Russia’s invasion of Ukraine, its first big test, the armed forces have floundered. Western intelligence estimated last week that 5,000 to 6,000 Russian troops had been killed, some of them poorly trained conscripts.

The dead included four Russian generals—one-fifth of the number estimated to be in Ukraine—along with other senior commanders, according to a Western official and Ukrainian military reports. The generals were close to the front lines, some Western officials said, a sign that lower ranks in forward units were likely unable to make decisions or fearful of advancing.

Russian troops turned to using open telephone and analog radios following the failure of encrypted communications systems, the Ukrainian Defense Ministry has said, making them vulnerable to intercept or jamming. Russian officers were likely targeted after their positions were exposed by their use of open communications, Western military analysts said.

In the strategically located town of Voznesensk, Ukrainian forces comprising local volunteers and the professional military drove off an attack early this month, in one of the most comprehensive routs Russian forces have suffered since invading Ukraine.

Russia’s failings appear to trace to factors ranging from the Kremlin’s wrong assumptions about Ukrainian resistance to the use of poorly motivated conscript soldiers. They suggest that Russia and the West overestimated Moscow’s overhauls of its armed forces, which some military analysts say appear to have been undermined by graft and misreporting.

The military’s previous outings in staged maneuvers and smaller operations in Syria didn’t prepare it for a multipronged attack into a country with a military fiercely defending its homeland, said Michael Kofman, director of Russia studies at CNA, a nonprofit research organization based in Arlington, Va.

“The failures that we’re seeing now is them having to work with a larger force than they’ve ever employed in real combat conditions as opposed to an exercise,” he said. “These exercises that we’ve been shown over the years are very scripted events and closer to theater than anything else.”

Destroyed Russian tanks after battles near Brovary, north of Kyiv.

Photo: Felipe Dana/Associated Press

Russia’s Defense Ministry didn’t respond to requests for comment on analyses of its performance. Russian President Vladimir Putin,

in an address to regional authorities on Wednesday, praised the war efforts, which in Russia are described only as a special military operation.

“The operation is being carried out successfully, strictly in accordance with previously laid-out plans,” he said. “And our boys and soldiers and officers are showing courage and heroism and are doing everything to avoid losses among the civilian population.”

Ukrainian President Volodymyr Zelensky last week said Ukraine had lost around 1,300 soldiers since the start of the invasion. A senior North Atlantic Treaty Organization official said losses were likely on par with the Russians’.

Insurgent tactics

For sure, Russia’s forces have taken territory, mainly in the south and east of the country against a smaller, less well-equipped adversary. Russian military commanders may also learn from their mistakes as they reposition their forces in readiness for a new offensive.

Western defense analysts say that even if Moscow’s military overcomes Ukraine’s armed forces eventually, they doubt that would end hostilities and merely mark the beginning of an insurgency that could tie up Russian forces for years. Moscow’s declared military objectives of replacing the government and establishing effective Russian control over a submissive population look remote.

But for now, Ukrainian forces have beaten back Russian paratroopers trying to secure airfields, and miles-long convoys of tanks and support trucks have stalled on highways out of fuel, Ukrainian soldiers’ videos and satellite imagery show. Hundreds of Russian military vehicles have been destroyed and others abandoned, sometimes because of mechanical breakdowns and poor-quality equipment, said Western officials and military analysts closely following the campaign.

Ukraine says its forces have downed more than 80 fixed-wing aircraft and 100 helicopters, though many fewer have been independently verified. Western officials have expressed surprise that Russia failed to use its superior air power to establish dominance of the skies, which left Ukraine’s much smaller air force operating.

Still, Russian warplanes flying over Ukraine continue to inflict heavy damage, including against civilians. The mayor of Mariupol said Russia’s air force had bombed the city’s drama theater Wednesday, killing an unknown number of people who had taken shelter there. Russia has denied responsibility. Mr. Zelensky in his video address to U.S. Congress on Wednesday said Ukraine is experiencing terror from the airstrikes every day, as he pressed for further military assistance.

Ukrainians have continued to attack long columns of Russian tanks and armored vehicles on open roads in formations making them vulnerable to Ukraine’s Turkish Bayraktar drones and its Territorial Defense units that use insurgent tactics to destroy fuel trucks, tanks and armored personnel carriers, videos posted by the Ukrainian military show.

In one such attack last week, Ukrainian drone footage posted on the Ukrainian armed forces’ YouTube channel showed the confusion caused by a Ukrainian ambush of a Russian column of dozens of tanks and armored vehicles approaching Brovary on the northeastern outskirts of Kyiv. The convoy suffers apparent drone hits at the front and the rear, trapping vehicles between them.

As soldiers escape their blazing vehicles, further explosions envelop them. Other tanks turn in panic, their tracks churning the road surface, before they retreat. Later footage shows tanks, apparently nearby, destroyed by an antitank weapon fired from a roadside position.

The movement of troops in bumper-to-bumper convoys is a clear sign of “soldiers who are untrained or undisciplined,” said retired U.S. Army Lt. Gen. Ben Hodges, a former commander of U.S. Army forces in Europe and now chair in strategic studies at the Center for European Policy Analysis in Washington. “You need sergeants or NCOs constantly telling them to spread out. It’s a human instinct to huddle together when you’re in danger,” he said. “I feel terrible for the young soldiers in the Russian army.”

A satellite image of a Russian convoy northeast of Ivankiv in February.

Photo: MAXAR/Associated Press

The NATO official said the Russians’ fighting style surprised Western observers because it didn’t follow the Russian military’s doctrine of using mobile units called battalion tactical groups and a consolidated system to command troops, which would have allowed the military to be nimbler against the enemy without extending supply lines dangerously inside Ukrainian territory.

“For now, they just can’t move,” the official said, adding that Russia has been trying to resupply the army by moving “trash”—civilian trucks and cars—across the country to the front line where they can be used by the military.

One central weakness of the campaign is Russia’s failure of coordination within and between branches of its armed forces, which has created problems in resupplying forces inside Ukraine and in coordinating the offensive, said military analysts including Andrew Monaghan, a Kennan Fellow at the Washington-based Wilson Center, a foreign-policy think tank.

“There are some tenets of Russian military history that feature in every war the Russians fight, regardless of whether they win or lose,” said Mr. Monaghan. “Those include considerable difficulties in complex operations, logistics, and always difficulties in command and control.”

Poor logistics have hindered the advance of the Russians, who use primarily railways to move men and materiel, the military analysts said. Russia’s failure to take the cities around Ukraine’s crucial railway junctions have forced the Russians to travel by road, leaving them easy targets.

Much of Russia’s military push was expected by military analysts to come into Kyiv from the North, where Russian troops have been exercising for months leading up to the invasion. But that push and others into the capital have slowed or stopped.

A Russian soldier captured on the outskirts of Kyiv, March 5.

Photo: Manu Brabo for The Wall Street Journal

Part of the problem has been the deployment of units from Russia’s eastern and central military districts, which have been secondary priorities in Russia’s military modernization, to advance on and surround Kyiv. The 90th tank division from the 41st combined arms army has brought with it less advanced T-72A and B tanks, designed in the 1980s, said Rob Lee, a senior fellow at the Foreign Policy Research Institute, a think tank.

The Russian invasion started to go awry from the first day. Following a morning barrage of rockets on Ukraine’s biggest airfields and military infrastructure on Feb. 24, Russia failed to push into Kyiv. An attempt to take the military airfield at Hostomel, some 20 miles outside of the city, if successful, would have given the Russians a foothold on the edge of the capital from which they could send in hundreds of paratroopers. Instead, an initial advance was pushed back.

Ukrainian units coordinated a multipronged attack on the first wave of Russian paratroopers that landed on the airfield, pushing them out and forcing the Russians to abort plans for more troop drops, according to firsthand reports of two soldiers involved in the fighting, official Ukrainian military reports and videos and images of events at the airfield.

Later, a National Guard, or Rosgvardia, column, sent from Russia’s loyal Chechnya region, was destroyed as it tried to retake the airport, Ukrainian authorities said. Chechen leader Ramzan Kadyrov said only two soldiers died.

The lack of coordination, Mr. Monaghan said, indicated the problems of controlling and coordinating military operations typical of the Russian military. Wars in the past, he said, had shown examples of commanders refusing to work with each other or come to each other’s aid, causing casualties, though there is no indication a lack of coordination at Hostomel was intentional.

“It doesn’t look as though the armed forces have communicated well with Rosgvardia,” he said.

Smoke rises near the town of Hostomel and Antonov Airport, in northwest Kyiv, in February.

Photo: daniel leal/Agence France-Presse/Getty Images

Flawed expectation

The initial invasion plan appears to have been to mount a lightning strike on Kyiv and quickly replace Mr. Zelensky’s pro-Western government with a Moscow-friendly administration. It was based on a flawed expectation of limited Ukrainian opposition, suggesting a failure of Russian intelligence or a reluctance to advise the Russian leadership that their forces should expect a hostile reaction, some Western officials have said.

Those sentiments have swelled since Moscow annexed Crimea and carved out two Russian-backed statelets in the east in 2014, from which Ukrainians have emigrated in thousands carrying stories of murders and political repression.

Those assumptions led to multiple errors that included only limited protection for invading troops, the NATO official said.

The Russians didn’t follow their own doctrine of launching the campaign with the Russian version of “shock and awe” under which they would have established superiority in the air and on the ground through rapid and massive deployment of weaponry. Instead, the Russians attempted to send in light forces deep into Ukraine that were poorly equipped to repel fierce resistance and far from supply lines.

A Russian serviceman’s body lies near destroyed Russian vehicles on the outskirts of Kharkiv on Feb. 26.

Photo: sergey bobok/AFP/Getty Images

Mr. Putin “really thought this was an illegitimate government that would quickly fall,” said CNA’s Mr. Kofman. “It’s clear he never left 2014, he really hasn’t understood anything that’s happened since 2014.”

Terror tactics

After failing in phase one, Russian tactics seem to have switched to terrorizing cities perhaps in search of a favorable political settlement.

Some military analysts expect the critical next phase to be around Kyiv where Russia likely plans to encircle the capital, blockade it and shell it into submission. This would replicate tactics used by the Russian army did during the second Chechen war that reasserted Moscow’s control over the predominantly Muslim statelet in 2000.

The Russians have moved from expensive guided-munition attacks to artillery attacks that have already gutted parts of the northern Ukrainian city of Kharkiv.

“It’s hard to say at this point whether that’s driven by expense, driven by a lack of inventory or a desire to be more brutal,” said the NATO official.

A residential apartment building in Kyiv damaged during a Russian strike.

Photo: Christopher Occhicone for The Wall Street Journal

Before they saw it in action, Western analysts had been largely impressed by Russia’s effort since 2008 to modernize its military. The overhaul emphasized a shift from large static units of a mainly conscript army to smaller, combat-ready groups of contract soldiers. It went alongside a modernization of equipment for land, sea, air and nuclear forces.

But the analysts overestimated the impact of new weapons such as battle tanks and jet fighters, said Phillips O’Brien, professor of strategic studies at the University of St. Andrews in Scotland. “Having weapons is only going to take you so far,” he said. “They have never had to operate these complex systems in this kind of conflict since at least the end of the Cold War, and you might actually say really since World War II.”

Russia—whose economy before the invasion was about the size of Italy’s—may have spread its efforts too thinly and the modernization effort also appears to have been undermined by fraud and corruption, said analysts including Michael Clarke, a former director of the Royal United Services Institute, a London think tank, and now associate director of the University of Exeter’s Strategy and Security Institute, citing estimates that some 25% of the invading force are conscripts.

Weapons systems haven’t performed well and commanders pretended they had capabilities that weren’t there, Mr. Clarke said. Of Russia’s effort to create a “large, modern army,” he said: “The part which is modern is not large, and the part which is large is not modern.”

—Daniel Michaels contributed to this article.

Write to Thomas Grove at thomas.grove@wsj.com and Stephen Fidler at stephen.fidler@wsj.com