As Russia steps up its offensive in eastern Ukraine, weeks of failed attacks on a Ukrainian stronghold have left two Russian brigades in tatters, raised questions about Moscow’s military tactics and renewed doubts about its ability to maintain sustained, large-scale ground assaults. The fighting has also come at a cost for Ukraine, which is expending vast amounts of ammunition to repel Russia’s growing numbers of ground troops, often supported by heavy armor, artillery and close air support. That has added urgency to Ukraine’s pleas for more ammunition, while Western allies this week expressed increasing concern about their ability to meet...