The counterattack began in the final days of August and at first focused on the southern region of Kherson, which was swept by Russian forces in the opening days of the invasion. But just as Moscow redirected attention and troops there, Ukraine launched another, highly effective offensive in the northeastern region of Kharkiv.

Facing the prospect of a large group of its forces becoming surrounded, Moscow ordered a troop pullback from Kharkiv, in a dramatic change of the state of play that posed the biggest challenge to the Kremlin since it launched the invasion Feb. 24.

“The Ukrainian army has taken advantage of the relocation of the bulk of the Russian forces to the south and is trying to direct the course of the war, excelling in maneuver and showing great ingenuity,” said Mykola Sunhurovskyi, a military expert with the Razumkov Centre, a Kyiv-based think tank. Ukraine’s quick gains, he added, are “important both for seizing initiative and raising troops’ spirit.”

Ukrainian President Volodymyr Zelenskyy commended the military in a video address late Saturday, saying it has reclaimed about 2,000 square kilometers (over 770 square miles) of territory so far this month. He also taunted Moscow over its withdrawal, saying the Russian army was “demonstrating the best it can do — showing its back” and “they made a good choice to run.”

Both sides have suffered heavy losses in Europe’s largest conflict since World War II. Ukraine’s military chief said last month that nearly 9,000 of the country’s soldiers have been killed in action. And while Moscow hasn’t reported its own losses since March, Western estimates put the toll as high as 25,000 dead, with the wounded, captured and deserters bringing the overall Russian losses to more than 80,000.

Ukraine has sought to mobilize the population to reach an active military of 1 million people, while Russia, in contrast, has continued to rely on a limited contingent of volunteers for fear that a mass mobilization could fuel discontent and upset internal stability.

As the war slogs on, a growing flow of Western weapons over the summer is now playing a key role in the counteroffensive, helping Ukraine significantly boost its precision strike capability.

Since the counteroffensive began, Ukraine said, its forces have reclaimed more than 30 settlements in the Kharkiv region.

In the Kherson region, troops sought to drive Russian forces from their foothold on the west bank of the Dnieper River, a potential vantage point for a push deeper into Ukraine by Moscow.

The city of Kherson, an economic hub at the confluence of the Dnieper and the Black Sea with a prewar population of about 300,000, was the first major population center to fall in the war.

Russian forces also have made inroads into the Zaporizhzhia region farther north, where they seized Europe’s largest nuclear power plant. The last of its six reactors was shut down Sunday after operating in a risky “island mode” for several days to generate electricity for the plant’s crucial coling systems after one of the power lines was restored.

Moscow has installed puppet administrations in occupied areas, introduced its currency, handed out Russian passports and prepared for local plebiscites to pave the way for annexation. But the counteroffensive has derailed those plans, with a top Moscow-backed official in Kherson saying the vote there needs to be put off.

The counterattack followed methodical strikes on Russian infrastructure and supply lines. Ukrainian forces have used American-supplied HIMARS multiple rocket launchers to pound the two bridges across the Dnieper, forcing Russian troops in the Kherson region to rely on pontoon crossings that also have faced daily strikes.

Last month, a series of explosions also hit airbases and a munitions depot in Crimea, underlining the vulnerability of the peninsula that was annexed by Russia in 2014 and has been crucial for its southern operations. Ukrainian authorities initially refrained from claiming responsibility, but the country’s military chief, Gen. Valerii Zaluzhnyy, acknowledged in recent days that his forces hit them with rockets.

Ukrainian military analyst Oleh Zhdanov said that “Ukraine has used the tactics of methodically exhausting the Russian army, weakening it and depriving it of a possibility to regularly beef up its forces.”

Unlike in the south, where Ukraine’s counteroffensive proceeded more slowly on the barren steppes of Kherson that left troops vulnerable to Russian artillery, the Kharkiv region’s forests offered natural cover that allowed for lightning-fast surprise attacks from multiple directions.

“Swiftness and surprise have become key components of the Ukrainian army action in the Kharkiv region after Russian forces deployed there had been relocated to the south,” Zhdanov said.

Michael Kofman, an expert on the Russian military at the Virginia-based think tank CNA, said the counteroffensive “has proven a very significant victory for Ukraine.”

“Russian forces appear to have been spread thinly, and military leadership unprepared despite earlier evidence of Ukrainian buildup,” Kofman wrote. “I think it’s fair to assess that Russia was caught by surprise with little in the way of reserves locally available.”

After capturing the town of Balakliia, about 55 kilometers (about 34 miles) southeast of Kharkiv, Ukrainian forces quickly pressed their offensive farther east to Kupiansk, a rail hub vital for sustaining Russian operations in the region.

They claimed control of the strategic city Saturday, cutting supply lines to a big group of Russian forces around Izyum to the south. To prevent their complete encirclement, Moscow ordered the hasty retreat, claiming that they were relocating to focus on the neighboring Donetsk region.

Zhdanov noted that a successful counteroffensive is key to persuading allies to further increase supplies of weapons to Ukraine, something that was discussed Thursday at a NATO meeting in Germany.

“The events in the south and in the Kharkiv region must show to the West that the Ukrainian military know to handle the weapons and need to develop their success,” Zhdanov said.

Yuras Karmanau in Kyiv contributed to this report.