Western weapons are now in action in the battle for Ukraine. But getting them into operation on the front lines is creating serious headaches for Ukraine’s military.

Ukrainian officials have depicted Western arms as essential in their efforts to turn the tide of the war against Russia, which has in recent months been making grinding territorial gains—and they want more. Until recently, Ukraine has been dependent on heavy weapons built or derived from Soviet-era systems, of which Russia has better equipment in much greater numbers.