The Ukrainian army’s counteroffensive around the city of Kharkiv in northeastern Ukraine starting on Sept. 6 destroyed half of the best tank division in the best tank army in the Russian armed forces. A hundred wrecked or captured tanks in a hundred furious hours. That’s how much destruction the Ukrainians inflicted on the Russian 4th Guards Tank Division, part of the elite 1st Guards Tank Army, the Russian army’s best armor formation. Now the 1st GTA is retreating north in order to preserve what remains of its front-line divisions. But the damage the tank army has suffered could have lasting...