With the collapse of the unified Roman empire came the revival of large fleet actions. The Byzantine navy, the largest Mediterranean war fleet throughout most of the early Middle Ages, employed crescent formations with the flagship in the center and the heavier ships at the horns of the formation, in order to turn the enemy's flanks. Similar tactics are believed to have been employed by the Arab fleets they frequently fought from the 7th century onwards. The Byzantines were the first to employ Greek fire, a highly effective incendiary liquid, as a naval weapon. It could be fired through a metal tube, or "siphon" mounted in the bows, similar to a modern flame thrower. The properties of Greek fire were close to that of napalm and was a key to several major Byzantine victories. By 835, a similar weapon had spread to the Arabs, who equipped "harraqas", "fireships", with it, though they never entirely managed to copy Greek fire, which was a closely guarded state secret.