After 800, Southeastern Hungary was conquered by Bulgaria. The Bulgarians lacked the power to establish effective control over Transylvania.^[12] Western Hungary (Pannonia) was a tributary to the Franks. Under the expansionist policy of the Kingdom of the East Franks, the rudimentary Slavic polities couldn't develop, except one, the Principality of Moravia, which was able to expand into modern-day Western Slovakia.^[13] In 839 the Slavic Balaton Principality was founded in southwestern Hungary (under Frank suzerainty). Pannonia remained under Frankish control until the Hungarian Conquest.^[14] Although diminished, the Avars continued the inhabit the Carpathian Basin. The most significant stock, however became the rapidly increasing Slavs^[15] who entered the territory mainly from the south.^[16]