Clad in mismatched fatigues, Ukrainian Marine recruits sprawled on the grass, cocking their assault rifles and aiming at targets. Then, to the surprise of their American instructors, one by one they started squeezing the trigger. “Cease fire!” yelled Steven Tomberlin, 62, a retired police officer from Colorado overseeing this part of the training. “Until I give the command. You. Do. Not. Do. Anything.” When the firing resumed, bullets hit the dirt berm, often far off the mark. “Most of these people have just been mobilized. They were electricians or tractor drivers yesterday, and many have never held a weapon in...