Ukraine’s Rout of Russian Forces Poses Challenge of How to Exploit Gains

Gen. Valeriy Zaluzhny, commander in chief of the Armed Forces of Ukraine, said the military had pushed Russian forces from around 1,150 square miles of territory—around the size of Rhode Island—to the east of Kharkiv, Ukraine’s second-largest city, since the start of September.

Now, Ukrainian commanders must decide how to exploit their battlefield success and prepare for Russia’s response.

Russia’s military said on Saturday that it was withdrawing from Kupyansk and Izyum, saying it was regrouping forces to defend Donetsk, which Moscow sent a covert proxy force to seize in 2014.

Even as Russian forces retrenched, they shelled parts of the Donetsk region they don’t control, killing 10 people in the city of Pokrovsk on Saturday, according to Donetsk Gov. Pavlo Kyrylenko.

North of Kharkiv, Ukrainian forces also appeared to be pushing toward the border with Russia. Vsevolod Kozhemiako, one of Ukraine’s leading businessmen turned commander, posted a video of himself near the border.

A battlefield map released by Russia’s Defense Ministry on Sunday appeared to indicate that Russian forces had vacated all northern parts of the Kharkiv region where Ukraine is continuing to press its counteroffensive and hasn’t claimed to have recaptured.

“These days, the Russian army is showing its best side—its back,” said Ukrainian President Volodymyr Zelensky in a statement on his Telegram channel.

The reversal came suddenly, following months of sputtering and ultimately stalled advances by the Russians behind withering artillery fire. After punching through a thinly defended Russian front line last week, Kyiv’s forces have raced forward, routing soldiers caught unprepared to fight. As panic spread in Russian-controlled settlements nearby, troops and occupants sympathetic to Moscow fled, in turn clearing a bigger path for advancing Ukrainian tanks and infantry.

The breakthrough is being hailed by Western military analysts as a great tactical maneuver. On social media, scenes abounded of ecstatic liberated Ukrainians greeting their troops, who ripped down the trappings of Russia’s monthslong occupation including flags and propaganda posters.

Ukraine’s speedy advance helps its cause on many fronts, starting with a morale swing in its favor. Energized, motivated advancing troops tend to fight better than demoralized, retreating defenders. The high-speed maneuver encircled potentially tens of thousands of Russian troops over recent days, Western analysts said.

The attack also appears to be bolstering Ukraine’s armories. Russian troops caught off guard abandoned ammunition and weapons from rifles to tanks and artillery pieces, according to images and reports on social media. They may also have left maps and documents that Ukrainian and Western intelligence analysts will mine for insights into Russia’s plans, tactics and weaknesses. After Russian forces retreated from around Kyiv earlier this year, abandoning camps and equipment, they left behind materials that proved valuable to Ukrainian analysts.

As Kyiv’s forces push east, more Russian targets will fall within range of Ukraine’s advanced Western rocket launchers, such as highly mobile Himars systems, which Kyiv didn’t have as it retreated earlier this year from the regions it is now retaking. Ukraine has used the precision munitions to destroy Russian supplies and air-defense systems, and the recent ground advances put more Russian-controlled territory within their range.

Outside Ukraine, its success in retaking within days territory that Russian forces spent weeks struggling to gain will strengthen Kyiv’s argument that it deserves armaments, funding and moral support from the West. Mr. Zelensky has repeatedly said his country can defeat and evict Russian forces, and now that idea appears less aspirational.

While Ukraine remains a long way from winning, its improved battlefield position strengthens its hand in any eventual peace negotiations with Russia.

Mr. Putin has yet to respond to Russia’s losses. Whether the defeats make him more or less threatening to Ukraine and the West remains an open question.

Over recent weeks he has increased economic pressure on Europe by cutting off already dwindling natural-gas flows through the Nord Stream pipeline to Germany and threatened to curtail grain exports from Ukraine.

“Western leaders in Europe have got to be able to say to their people: This is a miserable winter, but after this it gets easier,” said Michael Clarke, a professor of war studies at the University of Exeter in the U.K. “This is Putin’s last best shot, so if we can last out this winter and the Ukrainians look like they can kick the Russians out, things will turn against Putin.”

Ukrainian commanders now need to assess how hard to push ahead. Military history is littered with examples of armies that raced forward only to be blocked or repulsed.

“It’s great to make rapid gains, but the challenge is not to get your troops spread out too far,” said Jeffrey Edmonds, a specialist in national security and Russia and a senior analyst at the Center for a New American Security, a Washington think tank.

Ukrainian forces have the advantage of retaking friendly territory against a retreat, so defending their flanks and rear will be less of a concern than when punching into hostile terrain. Russia’s initial attack on Kyiv in February failed because its troops sped far ahead of their supply lines, leaving soldiers poorly provisioned. That weakened them, opening vulnerabilities to Ukrainian attacks from all directions.

But even thinly opposed, a charging force requires fuel, ammunition and food that can keep pace. After the Allied invasion of Normandy in 1944, U.S. Army Gen. George S. Patton’s troops raced more than 700 miles to Luxembourg within weeks, but his advance was halted in part because he had outrun supply lines.

“Logistics is the lifeblood of any military,” said Seth Jones, director of the international security program at the Center for Strategic and International Studies, a think tank in Washington. “Outrunning logistics is a challenge.”

Ukraine accomplished its breakthrough using deceit and surprise. Earlier this summer Mr. Zelensky announced a new offensive on the city of Kherson, in the southeast, and his troops began targeting Russian forces nearby. Russia responded by shifting forces there from Donbas.

Once Russia had relocated the troops, Ukraine last week struck the thinned-out Donbas defenders with its biggest massing of troops, tanks and other weaponry yet in the war. Some military analysts compared Ukraine’s counterattack to Israel’s furtive crossing of the Suez Canal during the Yom Kippur War in 1973 that allowed it to turn the tide in that fight with Arab neighbors.

Around Kherson, meanwhile, as many as 20,000 of Moscow’s troops are defending an occupied city that has now been cut off from resupply by Ukrainian strikes on bridges to their rear. Those bridges across the Dnipro River would also be needed for a Russian escape. Ukrainian troops aren’t advancing aggressively on the city, but time may be on Kyiv’s side in the siege.

—Isabel Coles contributed to this article.

Write to Daniel Michaels at daniel.michaels@wsj.com and James Marson at james.marson@wsj.com