Sigismund Augustus continued a tolerance policy towards minorities and maintained peaceful relations with neighbouring countries, with the exception of the Northern Seven Years' War which aimed to secure Baltic trade.

Under his patronage, culture flourished in Poland; he was a collector of tapestries from the Low Countries and collected military memorabilia as well as swords, armours and jewellery.

He was then involved in several relationships with mistresses, the most famous being Barbara Radziwiłł, who became Sigismund's second wife and Queen of Poland in spite of his mother's disapproval.

Sigismund Augustus was indifferent to her health condition; when the seizures continued to intensify he abandoned Elizabeth and returned to Kraków to collect her dowry.

The agitation was also abundant at Sigismund's first Sejm (parliament) sitting on 31 October 1548 where the deputies threatened to renounce their allegiance unless the new king repudiated Barbara.

By 1550, when Sigismund summoned his second Sejm, the nobles had begun to be in his favor; the nobility was rebuked by Marshal Piotr Kmita Sobieński, who accused them of attempting to unduly diminish the legislative prerogatives of the Polish Crown.

[9][10] While still married to Elizabeth, Sigismund Augustus ordered the construction of a secret passage connecting the Royal Castle in Vilnius with the nearby Radziwiłł Palace so that the couple could meet frequently and discreetly.

To ease the situation, Sigismund Augustus provided a luxurious lifestyle and expensive gifts for his wife at Wawel Castle since her arrival in Kraków on 13 February 1549.

He personally tended to his sick wife despite her foul smell and dedicated himself when necessary; the king hoped to take Barbara to the hunting castle at Niepołomice and ordered to demolish the small city gate so her carriage could pass freely.

[14] The death of Queen Barbara Radziwiłł, five months after her coronation and under distressing circumstances, compelled Sigismund to contract a third, purely political union with his first cousin, the Austrian archduchess Catherine, to avoid an Austro-Russian alliance.

Since her coronation, Catherine acted as Austria's puppet at the Polish court; she was tasked with espionage and obtaining important information for the benefit of the Habsburgs.

[18] Being involved in many affairs and holding a large number of mistresses, historians agree that the king had venereal disease which caused him to have fertility issues.

Great Marshal Jan Firlej denied these claims and reported that the king was bleeding severely due to consumption and was troubled by pain in the chest and lumbar.

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.

He managed to obtain more funds from the Sejm than his father ever could, and at one of the parliament sittings he won the hearts of the assembled envoys by unexpectedly appearing in a simple grey coat of a Mazovian lord.

His father, Sigismund I, permitted Albert of Prussia to introduce the Protestant Reformation and secularize the southern part of the Teutonic Order State.

As Prussia was a tributary state of the Polish Crown, Sigismund Augustus, a Catholic, was forced to intervene in favour of Protestant Albert and his brother Wilhelm.

Sigismund directed the alliance against Ivan the Terrible to protect lucrative trade routes in Livonia, thus creating a new valid casus belli against the Russian Tsardom.

Most of the deputies in the city council were merchants and tradesmen of German descent or Protestants who were either politically leaning towards Sweden or fighting for the status of an independent 'city state'.

The war ended as status quo ante bellum in 1570 with the Treaty of Stettin, which was signed by Bishop Martin Kromer on behalf of Sigismund Augustus.

Sigismund's most striking legacy may have been the Union of Lublin, which united Poland and Lithuania into one state, the Polish–Lithuanian Commonwealth, jointly with German-speaking Royal Prussia and Prussian cities.

Lithuanian magnates were afraid of losing their powers, since the proposed union would lower their rank and status to an equivalent with petty nobility rather than wealthier Polish aristocracy.

The most vocal opponent of the union was Sigismund's brother-in-law, Mikołaj "the Red" Radziwiłł (Lithuanian: Radvila Rudasis), who viewed the agreement as "peaceful annexation of Lithuania" by Poland.

As another war with Russia loomed, Sigismund Augustus pressed the members of parliament (Sejm) for the union, gradually gaining more followers due to his persuasive abilities and auspicious diplomacy.

Following Mikołaj Radziwiłł's departure from Lublin on 1 March 1569, Sigismund announced the incorporation of then-Lithuanian Podlachia, Volhynia, Podolia and Kiev provinces into Poland, with strong approval from the local Ruthenian (Ukrainian) gentry.

Now, under the Union of Lublin, all Ukrainian and Ruthenian territories which were alien in culture, customs, religion and language to the Polish people would be annexed by Catholic Poland.

Strong westernisation and polonisation would follow, including the clandestine suppression of the Ukrainian Eastern Orthodox Church by future King Sigismund III.

Calvinism became especially popular among the upper classes as it promoted democratic freedoms and called for rebellion against absolutism, which the privileged Polish nobility favoured.

During the 1555 Sejm session in Piotrków, the nobles intensively discussed the rights of priests in the newly proposed Polish Church and demanded the abolition of celibacy.

Despite this, Protestantism in Poland ultimately declined during the fierce Counter-Reformation measures under the despotic and arch-Catholic Sigismund III Vasa, who ruled for nearly 45 years.