Sigismund II Augustus I (Zygmunt II August, Ruthenian: Żygimont III Awgust I, Žygimantas III Augustas I, Sigismund II. August; 1 August 1520 — 7 July 1572) was King of Poland and Grand Duke of Lithuania, the only son of Sigismund I the Old, whom Sigismund II succeeded in 1548. Married three times, the last of the Jagiellons remained childless, and thus the Union of Lublin introduced Elective monarchy.
From the outset of his reign, Sigismund came into collision with the country's nobility, who had already begun curtailing the power of the great families. The ostensible cause of the nobility's animosity to the King was his second marriage, secretly contracted before his accession to the throne, with the beautiful Lithuanian Calvinist, Barbara Radziwiłł, daughter of Hetman Jerzy Radziwiłł.
But the real forces behind the movement seem to have been the Austrian court and Sigismund's own mother, Bona Sforza, and so violent was the agitation at Sigismund's first sejm (October 31, 1548) that the deputies threatened to renounce their allegiance unless the King repudiated his wife Barbara. He refused, and his moral courage and political dexterity won the day.
By 1550, when Sigismund summoned his second Sejm, a reaction had begun in his favor, and the nobility was rebuked by Piotr Kmita, Marshal of the Sejm, who accused them of attempting to unduly diminish the legislative prerogatives of the crown.
The death of Queen Barbara, five months after her coronation (December 7, 1550), under distressing circumstances which led to a suspicion that she had been poisoned by Bona Sforza, compelled Sigismund to contract a third, purely political union with his first cousin, the Austrian archduchess Catherine, also the sister of his first wife, Elisabeth, who had died within a year of her marriage to him, while he was still only crown prince.
The third bride was sickly and unsympathetic, and Sigismund soon lost all hope of children by her — to his despair, for as he was the last male Jagiellon in the direct line, the dynasty was threatened with extinction. He sought to remedy this by liaisons with two of the most beautiful of his countrywomen, Barbara Giżanka and Anna Zajączkowska. The sejm was willing to legitimatize, and acknowledge as Sigismund's successor, any male heir who might be born to him; however, the King was to die childless.
The King's marriage was a matter of great political import to Protestants and Catholics alike. Had Sigismund not been so good a Catholic, he might have imitated Henry VIII of England by pleading that his detested third wife was the sister of his first wife, and that consequently the union was uncanonical. The Polish Protestants hoped that he would do so and thus bring about a breach with Rome at the very crisis of the religious struggle in Poland; while the Habsburgs, who coveted the Polish throne, raised every obstacle to the childless King's remarriage.
Sigismund's reign was a period of internal turmoil and external expansion.
He saw the invasion of Poland by the Reformation, and the peero-cratic upheaval that placed all political power in the hands of the nobility; he saw the collapse of the Knights of the Sword in the north (which led to the Commonwealth's acquisition of Livonia) and the consolidation of Turkey's power in the south. Throughout this perilous transitional period, Sigismund successfully steered the ship of state amid the whirlpools that constantly threatened to engulf it. A less imposing figure than his father, the elegant and refined Sigismund II Augustus was nevertheless an even greater statesman than the stern and majestic Sigismund I the Old.
Sigismund II possessed to a high degree the tenacity and patience that seem to have characterized all the Jagiellons, and he added to these qualities a dexterity and diplomatic finesse which he may have inherited from his Italian mother. No other Polish king seems to have so thoroughly understood the nature of the Polish sejm. Both the Austrian ambassadors and the papal legates testify to the care with which he controlled his nation. Everything went as he wished, they said, because he seemed to know everything in advance. He managed to get more money than his father ever could, and at one of his sejms he won the hearts of the assembly by unexpectedly appearing before them in the simple grey coat of a Masovian lord. Like his father, a pro-Austrian by conviction, he contrived even in this respect to carry with him the nation, always distrustful of the Germans, and thus avoided serious complications with the dangerous Turks.
Sigismund II mediated for twenty years between the Roman Catholic Church and the Protestants without alienating the sympathies of either. His most striking memorial, however, may have been the Union of Lublin, which finally made of Poland and Lithuania one body politic, the Polish-Lithuanian Commonwealth — the "Republic of the Two Nations" (Rzeczpospolita Obojga Narodów, Abiejų Tautų Respublika). Also, German-speaking Royal Prussia and Prussian cities were included. This achievement might well have been impossible without Sigismund.
Sigismund died at his beloved Knyszyn on July 6, 1572, aged 52. In 1573, Henry III of Valois was elected as King of the Polish-Lithuanian commonwealth for a few months, but then returned to France where he was crowned as King Henry III of France. Shortly thereafter, Sigismund's sister Anna of Poland married Stefan Batory, and they ruled as King and Queen of Poland.
He married three times: