When a Russian tank army swept southwest from the Russia-Belarus border region into northern Ukraine on the morning of Feb. 24, a single Ukrainian tank brigade stood between it and the eastern suburbs of Kyiv. The Ukrainian army’s 1st Tank Brigade was outnumbered—not only by the 10 or so battalion tactical groups belonging to the Russian 41st Combined Arms Army, but also adjacent Russian formations. Incredibly, the 1st Tank Brigade not only halted the larger Russian force in the first few days of Russia’s wider war in Ukraine, it swiftly transitioned to an active defense and, for the next five...