Complex sentence: Buys became the head of the rebel'Raad van State' (one of the constitutional bodies of the Netherlands) in 1573, which would make him the rebel leader, if William of Orange died at the siege of Haarlem.
Split into simpler sentences: Buys became the head of the rebel'Raad van State' (one of the constitutional bodies of the Netherlands) in 1573. This meant that he would become the rebel leader, if prince William of Orange died at the siege of Haarlem.