They are from western Herzegovina, where ikavians are (not in Montenegro).
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They could be Vlachs who admixed with Zahumljani, Zahumlje correspond to western Herzegovina. But I am really not sure.
They became part of Croatian nation in 19th century, when modern South Slavic nations were formed. Before that people identified along religious lines, peasants had no sense of ethnic identity (only tribal in some cases), which was introduced to them by educated elites.
You could say that. But Bosnian muslims spoke ikavian in masses until recently. It was dominant dialect among them.
Among Serbs ikavian is present only in areas such as Kupres and small village or two in Dalmatian Hinterland where they were in contact with Croat majority.
DAI says Zahumljani were Serbs, but I don't think it's correct because their Prince mentions origins from tribe Litziki in Poland on Vistula river.
In my opinion Byzantine Emperor concluded they are Serbs because at time DAI was written this Sclavinia was under political control of Serbia.
Target: Feiichy_Father
Distance: 2.1956% / 2.19564487 | ADC: 0.5x
64.4 Croat_Split
23.2 Hungarian
12.4 Slovenian
Okay, just wanted to see where you stand on the issue.
In my opinion, historical sources are usually right unless contradicted/shown to be false.
Here are some other
sources that say Serbia extended past the Drina River (backing the DAI here):
https://i.imgur.com/SU1LBrc.png
Bosnia is mentioned as part of Serb even when not controlled by Serbia for example.
No other Slavic tribes were named around this time period. It seems unlikely they were neither Serbs or Croats or else we would've heard of them.