BUCHA, Ukraine — The Russian forces that were intent on overwhelming Kyiv at the war’s start with tanks and artillery retreated under fire across a broad front on Saturday, leaving behind them dead soldiers and burned vehicles, according to witnesses, Ukrainian officials, satellite images and military analysts. The withdrawal suggested the possibility of a major turn in the six-week war — the collapse, at least for now, of Russia’s initial attempt to seize Kyiv, the Ukrainian capital, and the end of its hopes for the quick subjugation of the nation. Moscow has described the withdrawal as a tactical move to...