In 1166, Stefan Nemanja assumed the throne, marking the beginning of a prospering Serbia, henceforth under the rule of the Nemanjić dynasty. Nemanja's son Rastko ( "Saint Sava"), gained autocephaly for the Serbian Church in 1219 and authored the oldest known constitution, and at the same time Stefan the First-Crowned established the Serbian Kingdom in 1217. Medieval Serbia reached its peak during the reign of Stefan Dušan (1331-1355), who took advantage of the Byzantine civil war and doubled the size of the state by conquering territories to the south and east at the expense of Byzantium, reaching as far as the Peloponnese, also being crowned Emperor of Serbs and Greeks in 1346.