WASHINGTON, March 1 - U.S. efforts to provide Ukraine weapons to battle the Russian invasion are complicated by limited supply lines, a potentially small window to save Kyiv from collapse and a Cold War legacy that made Western small arms ammunition incompatible with those produced in the Eastern Bloc.

While acknowledging the challenges moving weapons into Ukraine amid escalating Russian attacks, the Pentagon believes U.S. and Western assistance is proving effective. “We believe it is getting into the right hands, that they are actively using these systems,” a senior defense official said Tuesday.