6.2 argues that Slav migration from the north did happen, but the actual number of Slavic settlers was small and that the Illyrian ethnic substratum was prevalent for formation of Croatian ethnicity.
The Iranian origin of the Croats suggests that they are descendants of ancient Iranians (cf. Alans), these are perceived appearances of the name for Croatia or Croatians. The earliest claimed mention of the Croatian name, Horouathos, can be traced on two stone inscriptions in the Greek language and script, dating from around the year 200 AD, found in the seaport Tanais on the Azov sea, located on the Crimean peninsula (near the Black Sea). Both tablets are kept in an archaeological museum in Saint Petersburg, Russia. Whether the term Hourathos is related to the Croat ethnonym is open to conjecture, as the two words may have separate origins.
Genetically, on the Y chromosome line, a majority (>87%) of Croats belong to one of the three major European Y-DNA haplogroups -- Haplogroup I (38%), Haplogroup R1a 35% and Haplogroup R1b 16%. All three groups migrated to Europe during the upper paleolithic around 30,000-20,000 BC. Later, neolithic lineages, originating in the Middle East and that brought agriculture to Europe, are present in surprisingly low numbers. Haplogroup I is believed to have weathered the last glacial maximum in the southern Balkan peninsula itself, migrating north (including straight to modern day Croatia) as the ice sheets retreated.

See also: Medieval Croatian state and History of Croatia
The earliest Croatian state was the Principality of Dalmatia. Prince Trpimir of Dalmatia was called Duke of Croats in 852. The nationality of the Red Croats was vague at times, with the Neretvians accepting Croatian identity, while the Zachlumians maintained a Croatian identiy for some time.
In 925, Croatian Duke of Dalmatia Tomislav of Trpimir united all Croats. He organized a state by annexing the Principality of Pannonia as well as maintaining close ties with Pagania and Zahumlje.
Since the creation of the personal union with Hungary in 1102, the Croats were at times subjected to forceful Magyarization as well as, since 1527, Germanization. The ensuing Ottoman conquests and Habsburg domination broke the Croatian lands into disunity again—with the majority of Croats living in Croatia proper and Dalmatia. Large numbers of Croats also lived in Slavonia, Istria, Rijeka, Herzegovina and Bosnia. Over the centuries ensued a wave of Croatian emigrants, notably to Molise in Italy, Burgenland in Austria and eventually the United States of America.
After the First World War, most Croats were united within the Kingdom of Slovenes, Croats and Serbs, created by joining South Slavic lands under the former Austro-Hungarian rule with the Kingdom of Serbia, Croats became one of the constituent nations of the new kingdom. The state was transformed into the Kingdom of Yugoslavia in 1929 and the Croats were melted into the new nation with their neighbour fellow-South Slavs—Yugoslavs. In 1939, the Croats received a high degree of autonomy when the Banovina of Croatia was created, which united almost all ethnic Croatian territories within the Kingdom. In the Second World War, the Axis forces created a puppet state—the Independent State of Croatia, led by the fascist Ustaše movement, which sought to create an ethnically clean Croatian state. In response, many Croats joined the anti-fascist supra-ethnic partisan movement, led by the Communist Party of Yugoslavia. During and after the war, between 40,000 and 200,000 Croats lost their lives.
Post-war Socialist Federal Republic of Yugoslavia became a federation consisting of 6 republics, and Croats became one of two constituent peoples of two—Croatia and Bosnia and Herzegovina (in the latter one of the three since 1968). Croats in Serbia, in autonomous province of Vojvodina never reached that status. Following the democratization of society, accompanied with ethnic tensions that emerged in post-Tito era, in 1990 the Republic of Croatia declared independence, which was followed by war with its Serb minority, backed up by Serbia-controlled Yugoslav People's Army. In the first years of the war, over 200,000 Croats were displaced from their homes as a result of the military actions. In the peak of the fighting, around 550,000 ethnic Croats were displaced altogether during the Yugoslav wars.
During the Bosnian War, which followed the one in Croatia, the Bosnian-Herzegovinian Croats attempted their own independent state inside Bosnia and Herzegovina—the Croatian Community/Republic of Herzeg-Bosnia, but subsequently joined into the Federation of Bosnia and Herzegovina.
Post-war government's policy of easing the immigration of ethnic Croats from abroad encouraged a number of Croatian descendants to return to Croatia. The influx was increased by the arrival of Croatian refugees from Bosnia and Herzegovina and from Vojvodina (Bačka and especially Srijem). After the war's end in 1995, most Croatian refugees returned to their previous homes, while some (mostly Croat refugees from Bosnia-Herzegovina and Janjevci from Kosovo) moved into the formerly-held Serb apartments.

Main article: Culture of Croatia