BEIRUT — With international diplomacy in tatters and the U.S. focused on its election, the Syrian government and its Russian allies are seizing the moment to wage an all-out campaign to recapture Aleppo, unleashing the most destructive bombing of the past five years and pushing into the center of the Old City. Desperate residents describe horrific scenes in Syria’s largest city and onetime commercial center, with hospitals and underground shelters hit by indiscriminate air strikes that the United Nations said may amount to a war crime. Debris covers streets lined with bombed-out buildings, trapping people in their neighborhoods and hindering rescue workers. If government forces and their allies capture the rebel-held eastern neighborhoods, it would be a turning point in the 5½-year-old civil war that has killed more than 250,000 people and displaced half of Syria’s population.