The duration of conscription in Croatia will be two months from January 1st, while in Serbia it will be 75 days.
Hardly an indication of impending war.
This is even before one considers that Croatia is also a NATO member, which makes it quite unlikely Serbia would ever attempt to invade it.
Having a 75 day training period could mean several things:
1) There are not enough experienced officers in the Serbian military to provide the requisite number of people with six months or more of training. So, the only way to train X number of people per year is to shorten the length of training.
2) The military is unpopular enough, that it can no longer rely on volunteers, and conscripting people for more than 75 days is expected to lead to mass discontent.
3) The governments are playing at politics, angling for the votes of conservative Serbs and Croats. They do it in the cheapest and most meaningless way, but such that it still appears as though "Vucic is serious on defense!".
You can't train a modern soldier in two months. The results are visible for all to see in the war in Ukraine.
In two months you can only prepare a person for a very narrowly defined set of requirements. Like, how to storm a trench as part of a small infiltration group. The life expectancy of such a recruit is counted in days and weeks. I doubt that's what Serbia and Croatia are getting ready to do.
In Russia, mandatory service was reduced from two years to one year, about a decade ago. A lot of military officials now complain that this produces useless soldiers... so I can't imagine what kind of a soldier you get after 75 days.