One ruler of this nation as well as Hungary had hi
One ruler of this nation as well as Hungary had his head cut off as the result of an ill-conceived cavalry charge against the Janissary forces of Murad II at the Battle of Varna, while another ruler of it defeated Ulrich von Jungingen with the help of Vytautus the Great, stomping the Teutonic Knights at Tannenberg.  The Union of Lublin was signed at the instigation of another of its rulers, Sigismund II, as formal recognition of its union with Lithuania.  Kara Mustafa's attempts to take Vienna were thwarted by another ruler of this nation, Jan III Sobieski.  For 10 points, name this nation which was ruled by the Jagiellon dynasty and got partitioned a lot.
One ruler of this nation as well as Hungary had his head cut off as the result of an ill-conceived cavalry charge against the Janissary forces of Murad II at the Battle of Varna, while another ruler of it defeated Ulrich von Jungingen with the help of Vytautus the Great, stomping the Teutonic Knights at Tannenberg.  
Poland [accept Poland-Lithuania]