A 155th Mechanized Brigade tank crew in training.Ukrainian defense ministry photo A pair of Russian field armies, together overseeing 70,000 troops in dozens of regiments and brigades, is bearing down on Pokrovsk, a fortress city in eastern Ukraine’s Donetsk Oblast. Bracing for the coming assault, the culmination of a Russian offensive that began more than a year ago, the Ukrainians are reinforcing Pokrovsk. But one of the reinforcing units, the newly formed 155th Mechanized Brigade—one of the few Ukrainian brigades with German-made Leopard 2 tanks and French-made Caesar howitzers—began disintegrating before it even arrived in the besieged city last week.