But Cuius regio, eius religio is the law of the land in 1602, it has been since Charles V accepted it during the Diet of Augsburg in 1555. And at this date the estates of Bohemia can legally elect a different King that the Habsburg Archduke of Austria, thus it is not a seizure of that fief.
Not for all Protestant denominations though, Calvinism was only added in 1648.
It was an infringement on the rights of a fellow vassals. Ferdinand II had been elected as king of Bohemia in 1617, Frederick of the Palatinate was elected as anti-king of Bohemia in 1619.
Regardless other Protestant ánd Catholic Princes of the Holy Roman Empire are most likely to stay out of the conflict.