These people, constantly facing the Tatar warriors on the steppe frontier, received the Turkic name "Cossacks" ("Kazaks"), which was then extended to other free people in Russia. Many Cumans, who had assimilated Khazars, retreated to the Principality of Ryazan (Grand Duchy of Ryazan) after the Mongol invasion. The oldest mention in the annals is of Cossacks of the Russian principality of Ryazan serving the principality in the battle against the Tatars in 1444. In the 16th century, the Cossacks (primarily of Ryazan) were grouped in military and trading communities on the open steppe, and began to migrate into the area of the Don.