This struggle became a common cause of ours for the coming centuries from the appearance of the Ottomans on the Balkans. With our country torn three after the fall of Buda (1541) the Polish foreign politics dealt with two Hungarian states, the Kingdom with the Habsburgs, and the Principality of Transylvania with her changing ruling families. The relation of the Princes to the Sublime Porte varied during this era, some were more independent than others, some had better relation with the Habsburgs, some dreamed of winning the Kingdom for themselves. One however was approached by some representatives of the Sejm, they offered Anna Jagellion's hand in marriage to Stephen Báthory, and with her the power over of the Polish-Lithuanian Commonwealth. After him, the Princes saw the relations with Poland even more important if it's possible, but more importantly the interactions between the two nations (if I'm allowed to write this term somewhat anachronistically) started to change, from high politics it moved onto lower levels as well.
Gonna continue from here.