Moscow is deploying thousands of soldiers to southeastern Ukraine as it renews an assault on a strategically important town that Ukrainian forces have used to harass shipments on a critical Russian supply line that runs from the eastern Donbas region to Crimea.

The town, Vuhledar, has long been in Russia’s cross hairs. It sits at the intersection of the eastern front in the Donetsk region and the southern front in the Zaporizhzhia region, close to the only rail line linking Crimea with the Donbas region. The Ukrainians have used that proximity to lob artillery shells at the trains, limiting Russia’s ability to move men and equipment between the two fronts and, ultimately, to achieve its stated aim of capturing the Donbas, which comprises the provinces of Luhansk and Donetsk.

After a major drive in November failed, with reportedly enormous losses, Russian commanders are once again attacking in and around Vuhledar in hopes of securing the rail line.