TRIPOLI, Libya—Six weeks after the fall of Tripoli, the palmy days of rebel unity have begun to disintegrate into a spiral of infighting, political jockeying and even the occasional violent flare-up threatening to derail Libya's post-Gadhafi transition. Regional rivalries between fighters from the western mountains and Tripoli have in recent days come perilously close to exploding into open warfare in the capital. In some neighborhoods, multiple leaders claim sovereignty for their groups amid a deepening battle over the makeup of a citywide military council.