Israeli airstrikes flattened homes in Gaza on Saturday and rocket barrages into southern Israel persisted, raising fears of a renewed escalation in the longstanding conflict. The latest confrontation between Israel and Gaza militants is in its second day and has killed at least 15 people in the coastal strip. The fighting began with Israel’s killing of a senior commander of the Palestinian Islamic Jihad militant group in a wave of strikes Friday that Israel said were meant to prevent an imminent attack. On Saturday afternoon, Israeli warplanes stepped up strikes with hits on four residential b…