In 1569 the Lublin treaty wad signed between Poland and Lithuania, and the two states created a Commonwealth - the Rzecz Paspalitaya . The Polish nobility and the catholic priesthood were the initiators of the Union. The unified state structure was established in both states, nevertheless both Poland and Lithuania had internal autonomy. They had their own legislature, courts, financial system, military troops and the right to mint money. The states had to coordinate their international affairs. Lithuania had to join the Union because it was weakened by long wars with Moscovija. Poland wanted to solve the agrarian crisis at the expense of the lands of the GDL. The Lublin Union contributed to the Polish expansion on the Belarusian lands. The Poles got the right to buy land in the GDL, and by the end of the16th and the beginning of the 17th century Polish nobles owned whole quarters in Brest. Such quarters were called uridik. People living in such neighborhoods ceased to be under the jurisdiction of the city municipal and were placed under the control of the owner of the neighborhood. The development of trades contributed only to the enrichment of wealthy merchants and the church. They wanted to limit the rights of the city. After the Union, the Polish low-rank nobility (shlyakhta) flooded into Belarus buying its lands.

Along with the Polish nobility Roman catholic priest and Jesuits flooded into the GDL. Belarus faced both an economic and a religious challenges after the Jesuits had started promoting the catholic faith. The main Jesuits’ target were prominent nobles of Lithuania who possessed much land and peasants. Very often the Jesuits succeded. They intervened into the fields of science and education, developed missionary activity. By the 16th century in Brest the Collegiums and Orders of Jesuits and other servants of Vatican appeared. They were granted extensive land territories to build monasteries and churches. The property of Polish landowners and Catholic priesthood was not subject to taxation by the city. Polish Catholic influence was gradually increasing.

The decisive factor had been the acceptance of the Brest Church Union (1596), after which the authorities began to actually support the Uniates and the Catholics. Vatican wanted to create bridgehead for the promotion of the Roman Catholic faith on the Belarusian and Ukrainian lands. In 1596 in Brest an agreement was signed that united with the Roman Catholic Church several million Belarusian and Ukranian Orthodox Christians living under Polish rule in Lithuania. Inspired by the Council of Florence, which sought the reunion of all Eastern churches with Rome, the metropolitan of Kiev, Michael Ragoza, began negotiations with Catholic churchmen and the Polish king Sigismund III, a Roman Catholic. At a synod held in Brest, the Ukrainian Orthodox hierarchy declared their wish to submit to Rome. The Polish monarchy, fearful of Russian influence, particularly through its Orthodox Church, also thought to unify the various peoples under its rule through Catholicism. Hence the King was pleased, and he promised the Orthodox the rights and privileges enjoyed by the Latin rite as well as the preservation of traditional Eastern rites and customs. These guarantees were proclaimed by Sigismund on August 2, 1595; and in 1596 the terms of Pope Clement VIII and the King were accepted at another Orthodox synod at Brest, attended by the bishops of Vladimir, Lutsk, Polatsk, Pinsk, and Kholm, as well as the Metropolitan of Kiev. The synod divided into two factions. A peaceful reunion, however, did not result. Some delegates backed up the Union, the others, strong adherents of the Orthodox faith, came against the Union: the bishops of Lvov and Przemysl refused to comply. The Orthodox faction refused to sit with pro-Catholics and required to deprive all the adherents of the Union of holy orders. The King rejected the request of the orthodox faction, and in October 11, 1596 the Brest Union was concluded.

The Brest Union was buttressed by many Belarusian prominent landowners. At first times the common people also accepted the terms of the Union. Numerous Orthodox churches turned into the Uniate ones. The Bierascie Union started to become national Belarusian religion. Not everyone, though, accepted the Union. The Orthodox adherents viewed the Union as a threat for the Orthodox church. In 1620 the fight against the new religion flared up. That year the Constantinople patriarch Theophil visited Belarus on his return from Moscow, and he called to fight against the Union. Many people left the Uniate church and returned to the Orthodox one. Orthodox laymen founded brotherhoods to oppose the Union. The opponents of the Brest-Litovsk Union felt that their tradition and autonomy were being given away and feared that the Union would breed hybridism or the tendency toward Latinization and hence a betrayal of ancient and nationalistic tradition. The Jesuits played unseemly role in the promotion of the Union in Brest. They saw the church union as the mean to turn Or thodox faith into Catholic. The Jesuits closed Russian colleges and Orhodox churches, confiscated their property, destroyed many old Russian monuments. In the beginning of 17th century the Jesuits school was opened in Brest where the children of the Polish and local newly catholic landowners were studying.

The brotherhoods formed to oppose the Union started setting up their own schools. Brest owes its educational development in the 16-18th centuries to one of those schools. The Brest Brotherly School along with Lvov and Vilna was the biggest in Lithuania. Lavrenti Zuzanij Tustanovski and his brother Stephan were teaching in the school. Zuzanij was one of the activists of the reformation movement. Three years of his work in Brest brought Zuzanij a great reputation of pedagogist and social activist. His brother, Stephan, was a well-known preacher. He could preach in Belarusian, Polish, Church-Slavonic and Greek. He criticised the intrusion of the Catholic church into the Orthodox faith. The Polish king issued the order to catch Stephan Zuzanij and try him. The Brest Orthodox synod defended Stephan saying that all his books are in accordance with the church canon. Brest also remembers another thinker - Kazimir Lysczinski. In his treatise “About Inexistence of the God” this Jesuit theologist indicated that people created the God for themselves, not vice versa. This work enraged the churchmen who said that his doctrine shakes the basis of the Commonwealth and Catholic faith. His destiny was determined. In Warsaw, after being tortured, Lysczynski was burnt in the “cleansing fire”.

At the end of the 16th century the opposition against the Church Union took the form of an armed rebellion. Many peasants trying to escape from the Polish landowners’ oppression would flea to Zaporozhskaya Sech, place in Ukraine where Ukrainian and Belarusian peasants organized the Community of free cossacks. Cossacks defended the Orthodox Church and opposed the Brest Church Union very much. Very often they were making plundering raids into the Belarusian lands where they were killing Catholic priests, burning Catholic churches. These raids resulted in the decay of many cities of the Commonwealth. In 1648 the battle of Ukrainian cossacks headed by Bogdan Khmelnitski with Poles reached its apex. The rebellion spread in the south part of Belarus, thus, Brest was involved. The uprising in Brest was suppressed. The participants were annihilated and the city was completely destroyed. The fighting of cossacks continued and being unable to withstand the Polish troops the leader of the Ukrainian cossacks Bogdan Kmelnitski asked the Russian tsar Mikhail to help the Ukraine in fighting with Poles. Thus, Russia became involved in the war with the Commonwealth.

The military actions started in the summer of 1654 when Russian troops intruded into Belarusian territory. Peasants and tradesmen, not pleased with the Polish rule, at first supported them. The Russians were merciless towards Poles and Jews. Thousand of people of these nationalities were either killed or taken to the slavery. Muscovites were depredating cities and villages. They sat their mayors down to rule Belarusian cities and their ruling did not differ too much from that of the Poles. The Polish suppression was substituted by that of the Muscovites. Seeing such a situation, the Belarusian people, mostly tradesmen and gentry, started rebellions against the Muscovites which were swiftly suppressed.

In November 15, 1655 troops of Muscovites defeated the detachments of the magnat Sapega near Brest. By the end of 1655 Muscovites occupied all Belarus. The same year Sweden made use of the situation and waged a war against Poland. In 1657 the Swedish took Brest and plundered it. Only in 1661 the Polish troops retook the city. Rebellions of the population on the occupied territories and the overstreched army weakened Muscovites and they had to withdraw their troops from Belarus. The peace was restored in 1667 and the fighting sides signed a peace treaty at the city of Andrusovo according to which Belarus and Ukraine were divided. Part of Belarus - the Smalensk region and part of Ukraine together with Kyiv found themselves under Russian rule. Brest remained in the Commonwealth.

Having concluded the peace, the Poles continued the polonization of Belarus. In 1697 the Law was issued which forebod the use of the Belarusian language. The effect of the polonization process was dire to the future of Belarusian culture. In 1699 the Polish Sojm issued a law saying that people of the orthodox faith cannot be elected into governmental bodies. Thus, Belarusian people were deprived of independent political life in Belarus.

The war with Moscow and later with Sweden destroyed the economy of Brest. Trade was in decadence. The city fortifications were ruined, and there were no funds to reconstruct them. Only one stone building preserved in the fortifications, two big and three small cannons which lacked projectiles and artillerists.

The patience of Belarus was tested again in 1700 when the Polish king August II and the Russian tsar Peter I waged a war against Sweden to take that part of the Baltic territories which belonged to Sweden. The war lasted until 1721. Belarus again turned into a killing field. In 1705 Russian troops intruded into Belarus. In Brest food stores were set to provide the Russian army. In 1706 Peter I visited the city and the same year Swedish troops also paid a visit to the city and depredated it. In the war, Russia was the winner, acquiring almost all Baltic territories and becoming superpower in Europe.

Swedish Troops Siege Brest. 18th century engravery.

Brest started to revive after the war, and became the main port on the Western Boog river. In 1770s A. Tizengauz founded two enterprises: a mill with seven looms, and a metal factory. The enterprises used extensively water energy. The mill was located in Brest while the metal factory was built 40 kilometers from Brest. By that time scale it was a big enterprise producing 30 tons of metal per year.

In the meantime, the religious persecution of Belarusians in the Rzecz Paspalitaya continued. In 1718 the Parliament (Sojm) adopted a law aimed at the liquidation of the Orthodox Church in Belarus. The Russian emperor Peter I intervened into the internal affairs of Poland in order to protect the Orthodox faith. This did not sober the Poles. Meanwhile, many anti -Polish rebellions flared up in Belarus. The religious tensions continued, and it gave the Russian Empress Catherine II the pretext to intervene into the internal Polish affairs again. In 1768 she brought the troops to Warsaw, the Polish capital, and under this pressure the Parliament had to equalise the rights of Catholics and non-Catholics. Nevertheless, the tensions continued, weakening the Rzecz Paspalitaya, and in 1773 the Russian troops again arrived to defend the orthodox brothers. Also Prussian and Austrian troops appeared in Poland. As a reward for the established order, the three states took some Polish territory. Russia acquired Eastern Belarusian lands. The first division of Poland took place. This act sobered some statesmen, and in 1791 the Sojm adopted the Constitution which guaranteed some rights to the citizens of the Rzecz Paspalitaya. Some landowners, though, were not pleased with the Constitution, and they organized the so-called Targovice Confederation (after the name of the city Targovice where the organizers summoned). The branch of the Confederation was also in Brest. The organizers addressed Catherine II to intervene and bring back the old, pre-Constitutional order. The Empress quickly responded sending troops to Poland. Again, for the established order Russia acquired some more Belarusian lands. The Russian troops stayed in Warsaw. The rest of the lands were under Russian guardianship.

The patriots wanted to save the country, and in March 24, 1794 a rebellion flared up in Krakow, Poland. It was supported on all western Belarusian territories. The rebels were led by Tadeusz Kastuszko, prominent activist of the Polish national movement. The rebellion was suppressed in October 1794 by the great Russian general A.V. Suvorov who flooded with blood western Belarus. Having won over the rebels, Russia and Austria in 1795 fulfilled the last, third, division of Poland. Thus, Rzecz Pospolitaya ceased to exist. Brest found itself in the Russian empire.