The history of Macedonians has been shaped by population shifts and political developments in the southern Balkans, especially within the region of Macedonia. The ideas of separate Macedonian identity grew in significance after the First World War, both in Vardar and among the left-leaning diaspora in Bulgaria, and were endorsed by the Comintern. During the Second World War these ideas were supported by the Communist Partisans, but the decisive point in the ethnogenesis of this South Slavic people was the creation of the Socialist Republic of Macedonia after the World War II, as a new state in the framework of the Socialist Federal Republic of Yugoslavia. The region that today forms the country of North Macedonia has been inhabited since Paleolithic times. It occupies most of the ancient kingdom of Paionia and part of the territory of what was in antiquity Upper Macedonia (which coincides with some parts of today's southern Republic of North Macedonia), the region which became part of the kingdom of Macedon in the early 4th century BC. It was settled by the Paionians and Dardani, peoples of mixed Thraco-Illyrian origin. The Paionians founded several princedoms which coalesced into a kingdom centred in the central and upper reaches of the Vardar and Struma rivers until they were finally conquered by Philip II, father of Alexander the Great, in 358 BC. The Roman province of Macedonia was officially established in 146 BC. By the 4th century AD the Paionians had become fully Hellenized or Romanized and had lost their ethnic identity. During most of Late Antiquity and the early Middles ages, Macedonia (as a region) had been a province of the Byzantine Empire. In the 6th century AD, the part which today forms the Greek region of Macedonia was known as Macedonia Prima (first Macedonia), and contained the Empire's second largest city, Thessaloniki. The rest of the modern region (today's Republic of North Macedonia and Western Bulgaria) was known as Macedonia Salutaris.