Reading about the three main dialects of Serbo-Croatian on Wikipedia, it struck me that the origin of the Croatian national identity is quite unclear. Leaving aside the Slovenians for a moment, there seems to be a relatively clear origin of the Croatian, Bosnian and Serbian identities -- being Catholic, Muslim and Orthodox, respectively, and also the fact that they were often ruled by different foreign powers throughout their history.
However, with the Slovenians in the picture, it gets somewhat more unclear. One thing is that the Slovenians and the Croatians are both Catholic. But what raised this question for me in the first place has to do with language: of the three Serbo-Croatian dialects, all present in Croatia, one, Kajkavian is (according to Wikipedia) closer to Slovenian than to the other two dialects spoken in Croatia. Not just that, but Kajkavian appears to have been the dominant dialect in Croatia in the early days of nationalism in the region.
This leads to the question of how the idea of a Croatian nation of speakers of Kajkavian, Chakavian and Shtokavian was formed, instead of, say, a nation of speakers of Slovenian and Kajkavian (which seems somewhat natural given the similarities between the two languages), or even a national identity encompassing all Catholic speakers of Kajkavian, Chakavian, Shtokavian and Slovenian. I guess what I'm actually looking for is an overview of the origins of the Croatian and Slovene national identities, with some insight as to why they are different. What did the "Croats" have in common that led to the creation of a common national identity, that the "Slovenes" did not?
In early 19th century it was quite unclear what nations were forming, and more so what they included. Illyrian movement had currents within it that would include most all South-Slavs, from Slovenians to Bulgarians. Shtokavian was chosen in that context as a most likely option to unite all, but it would require Slovenians to abandon their language. There were but a few Slovene intellectuals ready for that, and in any case it is quite doubtful that such a project could be successful.
However, something similar was accomplished with Kajkavian. And, yes in regards to vocabulary it has more in common with Slovenian than Shtokavian, however in terms of sentence structure it is closer to the later. Most Croatian intellectuals of early 19th century opted for Shtokavian, with two possible, not incompatible, goals. At least, it was regarded as a best option to unite Croatian dialects, at most as a starting off point towards some larger South-Slav nation. That did however require that Kajkavci accept a different dialect as standard (Chakavians also, but it was quite some time before it was at all clear if Dalmatians would or should be part of Croat nation). Most famously Ljudevit Gaj, a Kajkavian, opted for Shtokavian, made an early grammar, published newspapers to promote it, etc.
Not all Kajkavians agreed, and there were two conflicting emerging national identities in Kajkavian areas - Horvatski and Hrvatski. Difference is rather slight, and both mean "Croatian", one said in Kajkavian, other in Shtokavian. However, those that considered Horvatski (Kajkavian) to be a proper Croatian identity were quite worried about losing themselves into wider Hrvatski nation, or even wider South-Slav one, while that option was still open. Politically, it prompted them to align themselves with Hungary, as a way to protect Horvatski from Hrvatski identity. Horvatska nation as it happen never emerged, so their worries would seem justified in the end. (As a curiosity, I've met some Kajkavians that today consider Gaj a traitor because of mostly lost Horvatska nation)
It is very likely that Kajkavian was integrated into Croatian due to being in the same political-administrative unit, due to Kajkavian area including Zagreb as a capital, and capital became a focal point for Shtokavian, and due to quite prominent Shtokavian movement among Kajkavians, mentioned Gaj being one, perhaps most prominent example. With that, inclusion of Slovenes would really mean an abandonment of their language, however national movement based on it was quite developed already by 1830-is or '40-is.
I know that Vuk Karadzic, 19th-century Serbian intellectual and language reformer, considered Shtokavian speakers (regardless of religion) to be "really Serbs" and Kajkavian speakers to be "really Slovenes", leaving the Croats with only the Chakavian speakers.
Also, it's worth noting that the Kajkavian area of modern Croatia was part of Hungary during the 19th-century, while modern Slovenia belonged to Austria. This may have had some effect on the development of national identities during the Austro-Hungarian period. Being ruled directly by Hungary really spurred the Slovaks to develop their own literary language and move away from the Czechs. It's possible that Kajkavian speakers found it more politically advantageous to associate themselves with Shtokavian speakers who were also part of the Kingdom of Hungary, rather than with the Slovene speakers in Austria.