Tartus is the second largest port in Syria, located on the Mediterranean just north of the border with Lebanon. Originally founded as the Phoenician colony of Aradus in the second millennium BC, it later fell under the sway of (successively) the Greeks, the Romans, and the Byzantines. It was brought into the expanding Muslim empire in 636. The city was captured by the Crusaders, who renamed it Tortosa. When the Knights Templar acquired it in 1152, they constructed a major castle surrounded by thick double concentric walls. The city was recaptured by Saladin in 1188, but the Knights continued to occupy the castle for another 100 years. Later, the city became part of the Ottoman Empire. When that empire fell with the end of World War I, Syria (including Tartus) came under French protection until independence was established in 1946. With a population of over 100,000, Tartus is an important commercial center. In the early 1970s, the former Soviet Union obtained basing rights at several locations in the Arab world. The naval supply and maintenance base in Tartus is the only remaining Russian military base outside the boundaries of the former Soviet Union. It consists of two floating piers, a floating workshop, storage facilities, barracks, and other facilities and is located just inside the northern breakwater of the port. It serves as a refueling, repair, replenishment, and liberty port for Russian naval vessels in the Mediterranean. The Russian naval facility in Tartus hosts the Amur-class floating workshop PM-138, which provides technical maintenance to visiting Russian warships. The naval base is also used as a transit point for shipment of Russian military and economic aid to the embattled Bashar al-Assad regime in Syria.