From the outside, this was shaping up as a strong year for wartime Ukraine. Ukrainian drones are burning Russia’s refineries and starving occupied Crimea of fuel. Moscow’s summer offensive has stalled on most parts of the front, and Ukraine is taking back territory. In Washington, a president who spent last year berating Kyiv has warmed to it again. But suddenly last week, President Volodymyr Zelensky dismantled what looked like a winning team — removing his popular defense minister, his prime minister and, late Tuesday, the acclaimed commander in chief of his armed forces. Zelensky, the comedian turned president who has...