Last fall, when the Ukrainian counteroffensive existed mostly on the maps and in the minds of its commanders, President Volodymyr Zelensky understood that its greatest obstacle could be the Dnipro River, especially as it flows through the southern city of Kherson and, about 50 miles upstream, fills the reservoir of a massive hydroelectric dam. “For us the Dnipro is the hardest story,” Zelensky told TIME during his visit to Kherson in November.