KUPIANSK, Ukraine —  

A tank carrying Ukrainian infantry speeds toward a target position marked with a metal sheet. The soldiers climb down, hurl grenades and unleash a crackle of machine-gun fire. Then they repeat the moves, getting faster with every iteration.

It’s only a drill. But with the sounds of the real war withi Russia rumbling four miles away, this daily training underscores the high stakes on Ukraine’s northeastern front, where military officials say a much-anticipated Russian offensive has already started, with fighting that could determine the next phase of the conflict.

Time is of the essence here, so speed and cohesion is the goal of the exercises that combine infantry assault and reserve tank units.

“Synchronization will be important to halt Russian offensives toward Ukrainian defensive lines,” said Ukrainian Col. Petro Skyba, a battalion commander of the 3rd Separate Tank Iron Brigade.

Grueling artillery battles have stepped up in recent weeks in the vicinity of Kupiansk, a strategic town on the eastern edge of Kharkiv province, by the banks of the Oskil River. The Russian attacks are part of an intensifying push to capture the entire industrial heartland known as the Donbas, which includes Donetsk and Luhansk provinces. It would be a badly needed victory for the Kremlin as the war enters its second year.Triumph in Kupiansk could decide future lines of attack for both sides: If Russia succeeds in pushing Ukrainian forces west of the river, it would clear the path for a significant offensive farther south where the borders of Luhansk and Donestk meet. If the Ukrainian defense holds up, it could reveal Russian vulnerabilities and enable a counteroffensive.Fighting on Ukraine’s northeastern front could determine the war’s next phase