When Ukraine’s long-awaited counteroffensive finally begins, the fight will be led by brigades armed not only with Western weapons but also Western know-how, gleaned from months of training aimed at transforming Ukraine’s military into a modern force skilled in NATO’s most advanced warfare tactics. As other Ukrainian units were fighting to expel the Russian occupiers from the country’s east and south, the brand-new 47th Separate Mechanized Brigade was preparing for the next phase of war from a classroom at a NATO base in Germany. The brigade’s leadership trained with computers that simulated situations they might face in real life. Deputy...