A headline deal between Libya’s prime minister and a leading military commander is a small step forward, but it leaves many problems unresolved.

In Paris in late July, a long-anticipated meeting between the head of Libya’s ruling executive and the leader of one of the country’s main armed groups finally took place. Under the precociously avuncular eye of French President Emmanuel Macron, the prime minister in Libya’s Presidency Council (PC), Fayez al-Serraj, reached...