Vladimir V. Putin exuded confidence. Sitting back, surrounded by foreign dignitaries, the Russian president explained the futility of Ukrainian resistance. Russia had the advantage on the battlefield, as he saw it, and by rejecting his demands, Ukraine risked even more for peace.“Keep at it, then, keep at it. It will only get worse,” Mr. Putin said at an economic forum in June, as he taunted the Ukrainian government. “Wherever a Russian soldier sets his foot, it’s ours,” he added, a smirk animating his face.His self-assurance is born out of the Russian military’s resurgence.In the depths of 2022, his underequipped forces were disoriented, decimated and struggling to counter Ukraine’s hit-and-run tactics and precision-guided weapons. Instead of abandoning the invasion, Mr. Put...