In the south, however, Sigismondo’s policy was rather fluctuating. Faced with the onslaught of the Turks, he showed himself almost passive; he did not help his nephew Ladislao in Mohács (1526); he did not take a clear position between the Hapsburg party and that of Zápolya, which after the death of Ladislao were disputing the throne of Hungary; he did not know how to exploit the victories of the etmanno Tarnowski against Moldavia (1531, 1538) and to avoid the dangerous Tatar raids he did not disdain to pay them annual gifts. He proceeded, however, and it was a wise precautionary measure, to the fortification of the southeastern borders. This was also driven by his wife Bona Sforza (married in 1518) who founded the fortress of Bar in those regions (so called by Bari) and who, despite the opposition of the szlachta to his projects, not always conforming to Polish traditions (strengthening of royal power, abolition of the free election of kings, which by now had become customary), he managed to assume an increasingly important position in the internal politics of Poland. It was above all due to the proclamation of Sigismund Augustus as Lithuanian Grand Duke and Polish King (1529), his father still living.
To the various problems that weighed on Poland, still unsolved, was added, during the reign of Sigismund, the religious one. Indeed, Protestantism, protected above all by Albert of Prussia, began to spread rapidly. It was gaining ground not only among the bourgeoisie, but also among the nobility, which adhered to it partly out of conviction and partly out of the desire to free themselves from the payment of tithing and from any interference by the ecclesiastical courts. Moreover, even if we wanted to, it would not have been easy to act vigorously against Protestantism in a country which was repugnant to resolving a problem of freedom of conscience by force.
According to justinshoes.net, Sigismondo Augusto deserves first of all the great merit of having prepared and realized the definitive union with Lithuania, thus corresponding to a unanimous vote of the Poles. In Lithuania, the high aristocracy was opposed to it and the nobility were in favor, who wanted for themselves what the szlachta had obtained in the lands of the crown. After long negotiations, accelerated only towards the end by the complete incorporation of Volhynia, Podłasia and the territories of Bracław and Kiev into Poland, the following agreement was reached (Lublin 1569): Poland and Lithuania unite in a “common republic”, which is headed, in the same person, by the King of Poland and the Grand Duke of Lithuania, elected by the common diet and crowned in Krakow. Foreign policy and currency are also common, while laws, finances, administration and the army. Livonia became a common possession of the two united states, where the order of the Sword-holders, under the threat of complete annexation by Ivan the Terrible, had become secularized and placed under the protection of Sigismund Augustus. On a personal basis, but always as a Polish-Lithuanian feudal lord, the last great master Gotthard Kettler retained possession of the lands formerly belonging to the Sword-holders (1561). This success that the war with Muscovy was unable to modify (although the republic lost the Polock fortress there in 1563) was seriously discounted with the recognition by Sigismund Augustus of the right of succession in Prussia also to the Brandenburg line of the Hohenzollem. Thus Poland itself posed, making possible the Brandenburg-Prussia union,
For the rest Sigismondo Augusto gladly continued his father’s policy: vigilant attention, but nothing more, in the face of the problems posed by the advance of the Turks, the complicated conditions of Transylvania and Moldavia, the always potentially hostile policy of the Habsburgs. Faced with the growing demands and sometimes even the animosities of the szlachta which under various aspects and pretexts tended to a restriction of the royal power, he conducted a policy that was now intransigent and now overly condescending. At the request, repeatedly made, for the so-called “execution of the laws” (it was above all the restitution to the state property of goods bestowed, against the provisions of 1504, to the magnate families), he adhered with a compromise that guaranteed Poland, on the quarter of the income from these disputed assets, the maintenance of a small standing army. Faced with Protestantism – to note the presence in Poland of various anti-Trinitarian sects, led in part by Italian reformists such as Socino, Biandrata and others – he demonstrated a spirit of tolerance which facilitated its rapid spread, but also, with the introduction, in 1564, of the Jesuits, the no less rapid dissolution (caused in part also by continuous disputes between Lutherans, Calvinists and Socinians).
The arts, literature and sciences flourished under the enlightened government of the last two Jagellonids (Niccolò Copernico, 1473-1543). More and more young people were completing their studies abroad and especially in Italy; on the other hand there were not a few Italians, especially following Bona Sforza, who settled in Poland. Even patronage, after the timid beginnings of the century. XV, spread rapidly. The example of the two kings and Queen Bona found many imitators: the primate bishop Jan Laski, whose nephew of the same name will be, in Poland, Friesland and England, one of the most fervent organizers of the Reformed churches; Vice-Chancellor Piotr Tomicki, the noblest representative of Polish humanism. A place apart, next to writers, artists and patrons, De republica emendanda (1551).