The subsequent war (see Livonian War) with Tsar Ivan IV the Terrible over Livonia compelled Sigismund to strengthen his position by constitutionally uniting all the lands attached to the Polish crown. Supported by the Polish and Lithuanian gentry, Sigismund ceded his hereditary rights in Lithuania to Poland (1564), thus placing the two states in constitutional equality but not in a complete union. In 1569 he formally incorporated Podlasie, Volhynia, and Kiev provinces into the Polish kingdom, thereby giving their representatives seats in the Sejm; the enlarged Sejm then enacted the Union of Lublin (1569), uniting Poland and Lithuania as well as their respective dependencies.