Great Moravia (Latin Moravia Magna) was a Slav state existing on the territory of present-day Moravia and Slovakia between 833 and the early 10th century. The first use of the designation "Great Moravia" stems from Constantine VII Porphyrogenitos in his work De Administrando Imperio (around 950 A.D.). "Moravia" did not refer (only) to present-day Moravia, but either to the country to both sides of all the Morava river, or probably to a state whose (today unknown) capital was called Morava. "Great" refers to Moravia plus the annexed territories.

A kind of predecessor of Great Moravia was the Empire of King Samo in around 623-658 in Moravia, Slovakia, Lower Austria (probably also Bohemia, Sorbia at the Elbe, and temporarily in Carinthia), which probably has not been a true state, but only a tribal union.

The development between 659 and late 8th century is largely unclear.

In the late 8th century, the Moravian basin, situated at an important north-south trade route, began to flourish. Two important states (principalities) emerged in this territory: the Moravian principality largely in present-day Moravia (led by prince Mojmír I, probable center: Mikulčice) and the Nitrian Principality (Principality of Nitra) in present-day western, central and northern eastern Slovakia (led by prince Pribina, center: Nitra).

What the historians and Porfyrogenet design as "Great" Moravia arose in 833 by Mojmír\'s conquest of the Nitrian Principality. The empire came under the rule of the Princes Mojmír I (833-846), Rastislav (846-870), Svatopluk (871-894) and Mojmír II (894-?) who built a great empire. Rastislav asked the Byzantine Emperor to send people who could interpret the teaching of Christ in the Slavic vernacular. Two of the people sent, Cyril and Methodius, laid the foundation of the Slavonic script, and thus of Slavonic literature (see e.g. Glagolitic alphabet).

The territory of Great Moravia was as follows:

  • 833 – 896/?907: today's Slovakia + Moravia + Lower Austria (territory north of the Danube)+ Hungary (territory north to Budapest and Theiss River, except for western Hungary)
  • 874 - ?: plus a strip of about 100km of present-day Poland above Slovak border (Vistula Basin, Krakow)
  • 880-? : plus a strip of about 100km of present-day Poland above Czech border (Silesia) and maybe also Great Poland
  • 880-896: plus remaining present-day Hungary east of the Danube
  • 880/883/884 – 894: plus the remaining present- day Hungary (up to Vienna)
  • 888/890 – 895: plus Bohemia
  • 890 – 897: plus Lusatia

After Svatopluk's death in 894, his 2 sons fall out with each other, thus weakening the empire. Invading Magyars (Hungarians), coming from Asia, destroyed the empire around 907 (However, there are historic references to Great Moravia from later years (e.g. 924/5, 942)). The western part of the core (=present-day Moravia) was annexed by Bohemia in 955 (very disputed), in 999 it was taken over by Poland under Boleslaus I of Poland and in 1019 it finally became part of Bohemia. As for the eastern part of the core (=present-day Slovakia) its southern parts were conquered by the Hungarians definitively in the 920's (western Slovakia maybe sharing the fate of Moravia from 955 to 999), in 1000 or 1001 entire Slovakia was taken over by Poland under Boleslaus I of Poland, Polish supremacy ended in 1025 or 1029, and in 1030 the southern half of Slovakia was again taken over by Hungary (remaining Slovakia was taken over by the Hungarians from the end of the 11th century till the 14th century).
The state has been a state of present-day Moravians and Slovaks. The western part of  Great Moravia's core (=present-day Moravia) was finally conquered by Bohemia in early 11th century and its population was czechicized in the 19th century. The eastern part of the core (=present-day Slovakia) was finally conquered by the  Magyars (Hungarians) in the 11th-14th century and its population developed into present-day Slovaks in the 10th century. Great Moravia is often considered a predecessor of Slovakia. 

The inhabitants of the core of the state were designed as "Slovieni" (which is an old Slavic word basically meaning "Slavs" and was also used by (future) Slovenians and Slavonians at that time) or "Moravian peoples" by Slavic texts, and as "Sclavi" (i.e. Slavs), "Winidi" (i.e. Slavs), "Moravian Slavs" or "Moravians" by Latin texts. The present-day terms "Slovaks" / "Slovakia" (in Slovak: Slováci / Slovensko) and "Slovenes" / "Slovenia" (in Slovene: Slovenci / Slovenija ) arose later from the above "Slovieni".

As for the history of Bohemia - annexed by Great Moravia 888/890-895—the important year is 895, when the Bohemians broke away from the empire and became Frankish vassals (vassals of Arnulf of Carinthia) and gradually an independent Bohemia, ruled by descendants of Premysl, began to emerge.