The history of the Cossacks spans several centuries.
Several theories speculate about the origins of the Cossacks. According to one theory, Cossacks have Slavic origins, while another theory states that the Constitution of Pylyp Orlyk of 1710 attests to Khazar origins. Modern scholars believe that Cossacks have both Slavic and Turkic origins. The Academician Ivan Zabelin mentioned that peoples of the prairies and of the woods had always needed "a live frontier", and even ancient Borisphenites (Dniepr Scythians) and Tanaites could be the predecessors of Cossacks, not only the Khazars, who assimilated/included Severians, Goths, Scythians and other ancient inhabitants, as insisted by Cossack folklore, by the Constitution of Pylyp Orlyk, and by numerous Cossack historians. Because of the need of both the Reds and the anti-Bolshevik forces to deny any separate Cossack ethnicity, the traditional post-imperial historiography dates the emergence of Cossacks to the 14th-15th centuries. Non-mainstream theories, however, have borrowed the date 948 from imperial historiography, and ascribe an earlier Cossack existence to the tenth century, but deny Cossack links both to "the old people" (Khazars) and to "the new people" (Russians and Ukrainians; the very terms "old people" and "new people" being coined by the 11th-century Metropolitan Ilarion of Kiev), specifically mentioning 948 as the year when the inhabitants of the steppe under a leader named Kasak or Kazak routed the Khazars in the area of modern Kuban and organized a state called Kazakia or Cossackia.
Cossacks were mainly East Slavs. In the 15th century, the term originally described semi-independent Tatar groups which lived on the Dnipro River, which flows through Ukraine, Russia and Belarus.
Some historians suggest that the Cossack people had mixed ethnic origins, descending from Russians, Khazars, Ukrainians, Tatars, and others who settled or passed through the vast steppe that stretches from Asia to southern Europe.