When Ukrainian forces hit a Russian tank with an American-made Javelin missile on a highway on the northern reaches of the capital, Kyiv, the explosion was so huge that it tossed the turret 10 yards down the road and shredded the rest of the vehicle and the men inside. The charred body parts of Russian soldiers were still decomposing on the asphalt three weeks later, alongside scattered metal debris and ammunition. A single felt boot, favorite footwear for centuries in Russia’s frozen villages, lay blackened among the detritus. “Good shot,” the Ukrainian commander said with elation, surveying the wreck. He...