Major questions about Ukraine’s artillery and other ammunition supplies remain. Kyiv’s supplies of air defense missiles and artillery rounds, critical to sustaining any push and to defend against Russian air attacks, could run dangerously low if its forces continue to expend ammunition at their current pace. After the offensive is over, there is little chance that the West can recreate the buildup that it did for Ukraine’s coming assault for the foreseeable future, because Western allies do not have enough supplies in existing inventories to draw from and domestic production will not be able to fill the gap until next year, experts say.

The Ukrainian military has been firing thousands of artillery shells a day as it tries to hold Bakhmut, a pace that American and European officials say is unsustainable and could jeopardize the coming offensive. The bombardment has been so intense that the Pentagon has raised concerns with officials in Kyiv, warning them that Ukraine was wasting ammunition at a key time.

While Ukrainian forces can use drones to strike behind Russian front lines, they have not been given missiles with a long-enough range to hit Russia’s logistical hubs, a tactic that proved important in last summer’s offensives outside Kharkiv and Kherson.

The Russians have challenges of their own.

Since the beginning of the invasion, there have been major doubts about the basic competence of Russian commanders and their supply of well-trained soldiers, artillery shells and equipment. The Russians have expended many of their cruise missiles, lost thousands of people in Bakhmut alone and drained their stores of ammunition much faster than they can replace them with their domestic production.