The wall of a major dam in a part of southern Ukraine that Moscow controls collapsed Tuesday after a reported explosion, sending water gushing downriver and prompting dire warnings of ecological disaster as officials from both sides in the war ordered residents to evacuate. Ukraine accused Russian forces of blowing up the dam and hydroelectric power station, while Russian officials blamed Ukrainian military strikes in the contested area. The fallout could have broad consequences: Flooding homes, streets and businesses downstream; depleting water levels upstream that help cool Europe’s largest nuclear power plant; and draining supplies of drinking water to the...