Tensions ran high on Wednesday after Fayez Serraj, a little-known Libyan technocrat selected as prime minister in a United Nations peace process, arrived by boat in Tripoli from Tunisia. Western officials hailed his installation in the Libyan capital as a sign that the country’s two-year political divide is finally coming to an end — despite the existence of rival governments in Tripoli and the country’s east.

The United States and European allies, including Italy, France and Britain, have made the unity government’s establishment a key precondition for launching twin missions to begin an international stabilization effort and help combat a growing Islamic State affiliate there.

Each of those tasks will be strained by tensions among militia factions that Western nations hope will form a unified front against terrorist groups and by strong reluctance among European nations to wade into Libya’s chaos — even among those countries most threatened by the Islamic State’s growth across the Mediterranean.