Croats and serbs
It’s impossible to answer, but there’s something very interesting: many foreigners have noticed that croats from slavonia (the northeastern region of croatia) and eg istria (the most western region) are barely the same people.
News world the bosnia crisis: serbs, croats and muslims: who hates who and why: tony barber in zagreb traces the ancient roots of a culture clash that has shattered what was yugoslavia into warring pieces. Serbs and croats assimilated or the official status of autochthonous national minority for the serbs of croatia is recognized by the croatian constitutional.
After the first world war and dissolution of austria-hungary, most croats were united within the kingdom of serbs, croats and slovenes.
Croats and serbs
Other answers are pretty good but not entirely true disclaimer: i'm a croatian at one point both croats and serbs had to be of the same origin, but it's probably so far in history that we don't have any written evidence of it, and at that point. Serbs although the serbs religion and because their dialect was similar to that of the serbs from the croatian military frontier roman catholic croats also.
The bitter and often violent contest in yugoslavia between serbs and croats, who share a common language and much common history, is of recent origin for most of 14 centuries these two slavic peoples lived in relative harmony as neighbors -- the serbs largely to the southeast and the croats largely. Amazoncom: serbs and croats: the struggle in yugoslavia (9780156806633): alex n dragnich: books. Serbs subsequently formed the kingdom of serbs, croats and slovenes with other south slavic peoples the country was later renamed the kingdom of yugoslavia.
Not as catholic serbs, no as catholic yugoslavs, perhaps part of the problem is terminology - part of the serbian/croatian identity is being orthodox/catholic. This topic is a complex one and for the most part is based on the abuse of power such abuse translated into promoting national interests, namely more.