Happy Birthday, Dmitry!
Today in 1350, one of the great Princes of Russia was born: Dmitry Donskoi (Dmitry of the Don). He built the first stone Moscow Kremlin (well, it was built under his reign), which allowed Moscow to withstand sieges by Algirdas of Lithuania, culminating in the Treaty of Lyubutsk. He also managed to settle things with both Vladimir and Nizhnyy Novgorod, as well as being acknowledged by other princes and dukes of the Rus, so that he more than doubled Muscovy's size.
But the main accomplishment of his thirty-year reign was the beginning of the end for Mongol domination. He defeated the general Mamai twice, at the Vozha River in 1378 and again in the decisive Battle of Kulikovo (on the Don River) in 1380. WHile it's true that Tokhtamysh, who replaced Mamai, reasserted Mongol rule over Muscovy, Dmitry pledged his loyalty to the Golden Horde and was reinstated as Mongol principal tax collector and Grand Duke of Vladimir. Upon his death in 1389, Dimitry was the first Grand Duke to bequeath his titles to his son Vasiliy without consulting the Khan.