Ukrainian forces on Sunday pushed their counteroffensive in the east of the country, capitalizing on rapid gains made in a week of fighting that has drastically changed the course of the conflict. Ukraine’s swift action to retake Russian-held areas in the northeastern region of Kharkiv forced Moscow to withdraw its troops to prevent encirclement and leave behind a significant number of weapons and ammunition in a hasty retreat as the war completed 200 days on Sunday. Jubilant Ukrainian President Volodymyr Zelenskyy taunted the Russians on camera late Saturday, saying “the Russian military these days is showing what it does best – showing its back.” On Sunday, he posted a video of Ukrainian soldiers raising the national flag over Chkalovsk, another town they captured from the Russians in the counterattack. Ukraine’s army chief, General Valerii Zaluzhnyy, said Sunday that Ukraine has liberated about 3,000 square kilometers (about 1,160 square miles) since early September. He noted that Ukrainian troops are now only 50 kilometers (about 30 miles) away from the border with Russia. The Russian retreat marked the biggest battlefield success for Ukrainian forces since they thwarted a Russian attempt to capture the capital, Kyiv, at the start of the nearly seven-month war. Ukraine’s offensive in the Kharkiv region came as a surprise to Moscow, which had moved many of its troops from the region to the south in anticipation of the main Ukrainian counteroffensive there. In an awkward attempt to save face, the Russian Defense Ministry said on Saturday that the withdrawal of troops from Izyum and other areas in the Kharkiv region was intended to bolster Russian forces in the neighboring Donetsk region to the south. The claim was similar to the justification Russia gave for withdrawing its forces from the Kiev region earlier this year after they failed to take the capital. The cluster of Russian forces around Izyum has been key to Moscow’s bid to seize the Donetsk region, and their withdrawal will now dramatically weaken Russia’s ability to attack the Ukrainian strongholds of Sloviansk and Kramatorsk just to the south. Igor Strelkov, who led the Russian-backed separatists in the first months of the Donbass conflict when it erupted in 2014, scoffed at the Russian Defense Ministry’s explanation for the retreat, suggesting that Russia’s territory near the border be handed over to Ukraine as ” contribution to the Ukrainian settlement”. The retreat drew angry comments from Russian military bloggers and nationalist commentators, who decried it as a major defeat and urged the Kremlin to respond by stepping up the war effort. Many sharply criticized Russian authorities for going ahead with fireworks and other lavish celebrations in Moscow that marked a city holiday on Saturday despite the devastation in Ukraine. As Russian forces hurriedly withdrew from Izyum under Ukrainian fire, Russian President Vladimir Putin attended the inauguration of a giant observation wheel in a Moscow park, a new transport link and a sports arena. The action underscored the Kremlin’s attempt to continue to pretend that the war it calls a “special military operation” was proceeding according to plan without affecting the situation in the country. Pro-Kremlin political analyst Sergei Markov criticized the celebrations in Moscow as a grave political mistake. “Fireworks in Moscow on a tragic day of Russia’s military defeat will have extremely serious political consequences,” Markov wrote on his messaging app’s channel. “Authorities should not celebrate when people mourn.” In a sign of a possible rift in Russian leadership, Ramzan Kadyrov, the Kremlin-backed leader of Chechnya, said the retreat from the Kharkiv region stemmed from blunders by the Russian military leadership. “They have made mistakes and I think they will draw the necessary conclusions,” Kadyrov said. “If they do not make changes in the strategy of conducting the special military operation in the next day or two, I will be forced to contact the leadership of the Ministry of Defense and the leadership of the country to explain the real situation on the ground. . Ukrainian Defense Minister Oleksii Reznikov said in televised comments on Saturday that the Russians had been cut off from supply lines and predicted more gains. “It will be like an avalanche,” he said, predicting a Russian comeback. “A line of defense will shake and fall.” Despite Ukraine’s gains, US Secretary of State Anthony Blinken and the head of NATO warned on Friday that the war would likely drag on for months. Blinken said the conflict was entering a critical period and urged Western backers of Ukraine to continue their support through a difficult winter. In another major development on Sunday, the Zaporizhzhia nuclear power plant, Europe’s largest, was reconnected to Ukraine’s electricity grid, allowing engineers to shut down its last operating reactor in a bid to avoid a radioactive disaster as fighting rages in region. For the past several days, the plant had been operating in “island mode” with only one of its six reactors working to power cooling systems and other critical equipment.