Great Moravia was a Slavic state that existed in Central Europe from the 9th century to the early 10th century. There is some controversy as to the actual location of its core territory. According to mainstream historians, its core territory laid on both sides of the Morava river, in present-day Slovakia and the Czech Republic, but the empire also extended into what are today parts of Hungary, Romania, Poland, Austria, Germany, Serbia, Slovenia, Croatia. This theory also states that Great Moravia was inhabited by the ancestors of modern Moravians and Slovaks. According to alternate theories, the core territory of Great Moravia was situated South of the Danube river, in Slavonia or in the southern parts of the Carpathian Basin.
Great Moravia was founded when Mojmír I unified by force two neighboring states, referred to by the modern historiography as the "Principality of Nitra" and the "Principality of Moravia", in 833. The rulers of the emerging state periodically accepted the supremacy of the Kings of East Francia, but they continuously endeavored to strengthen the independent status of their country.
Unprecedented cultural development resulted from the mission of Saints Cyril and Methodius, who came during the reign of Prince Rastislav in 863. The empire reached its greatest territorial extent under Svatopluk I (871–894), although the borders of his dominions are still under debate. He also achieved to have his independent status acknowledged by Pope John VIII who styled Svatopluk "king" in a letter.