There exist several hypotheses on the date and historical context of the migration to the Adriatic Sea, most often being related to the Pannonian Avars activity in late 6th and early 7th century. It is considered that the uprising happened after failed Siege of Constantinople (626), in the period of the Slavic uprising led by Samo against the Avars in 632, or 635-641 when the Avars were defeated by Kubrat of the Bulgars, which are also interpreted as revolts when were already settled. As the Avars were enemies of the Byzantine Empire the involvement of Emperor Heraclius on the side of Croats cannot be entirely excluded. It is also theorized that the migration of the Croatian tribes in the 7th century was the second and final Slavic migratory wave to the Balkans, which is related to the thesis by Bogo Grafenauer about the double migration of Slavs. According to this thesis, although it is possible that some Croatian tribes were present among Slavs in the first Slavic-Avar wave in the 6th century, it is argued that the Croatian migration, seen as of a warrior group, in the second wave probably was not equally numerous to make a significant common-linguistical influence into already present Slavs and natives, while others considered they arrived in a significantly larger number. However the thesis on dual division and migration is criticized for being unnatural and improbable.