Complex sentence: He came into the throne by championing the Protestant cause during the increasingly tense times of religious strife between competing sects of Christianity, which, in just over a decade, would break out as the Thirty Years' War.
Split into simpler sentences: He came into the throne by championing the Protestant cause during the increasingly tense times of religious strife between competing sects of Christianity. In just over a decade, these would break out as the Thirty Years' War.