In 1949, Benjamin Rivlin wrote an instructive piece “Unity and Nationalism in Libya” (JSTOR), excerpt:
…the Big Four have been sharply divided on the question of Libyan unity…In supporting the Sanusi claims, Great Britain has become the chief advocate of a divided Libya…Similarly, the United States has given support to a divided Libya by abandoning its original proposal for an international trusteeship, in favor of support for the British position…Not to be forgotten is…France, also, advocated a partitioning of Libya, but a partition of its own special variety. Under the guise of “border rectifications,” France has laid claim to the Fezzan in southwestern Tripolitania and to all of Libya south of the Tropic of Cancer…The French claim is based primarily on the fact that Free French troops wrested this desert region from Italian control, and is an attempt to bolster the sagging prestige of France as a world power by a tangible reward for its role in the war.
The Soviet Union opposed a partition of Libya and favored Italian trusteeship. Back then, it seems that Europe took the lead role and the U.S. followed along. Here is one good sentence:
In examining the history of Libya one is struck with the fact that only on rare occasions has the area constituted a unified political entitity…there have never been firm bonds of union.
The difference between the two territories goes back to antiquity, when the territory was divided by rule by Greece and rule by Phoenicia. Even when Italy claimed the country in 1912, it effectively governed over two separate territories, Tripolitania and Cyrenaica. What is the fundamental principle of division?:
The division of Libya into Cyrenaica and Tripolitania down through the ages is no mere quirk of history. It reflects, rather, the basic physiographic character of the territory. A great natural barrier — the Gulf of Sirte [now Sidra] and the projection of Libyan desert along its 400-mile shore — divides Cyrenaica from Tripolitania, limiting communication between the two territories and to a very large extent shaping their economies. Trade between the two territories has played a minor role, and the movement of the nomadic tribes in both territories has been and remains north-south, not east-west.
Unity vs. separatism has been the chief concern of all political leaders in Tripolitania and Cyrenaica ever since the relaxation of military administration controls during the past three years…
Here is a summary of the Sanusi. Here is a useful map. Having read this article, I have revised upwards my priors on the likelihood of partition as the result of the current conflict, whether or not Gaddafi falls.