The Lithuanian princes were reasonable rulers; in some cases they assimilated adopted local customs, language and religion. People did not resist them and appreciated their protection from Poland, Moscow and Tatars. However, under Polish rule, western Ukraine was subjected to exploitation and colonization by influx of people from Poland and Germany, who were taking over property and offices from local boyars.
There was a period of wars between Poland and Lithuania, but on 15th August 1385 they agreed to unite their kingdoms. In 1386 Polish queen Yadwiga was forced to marry Lithuanian prince Yahaylo, who thus became King of Poland and Lithuania.
In 1400 Lithuania, together with its Ukrainian principalities, separated under king Vitowt Yahaylo's cousin. This arrangement was opposed by Yahaylo's younger brother, Svytryhaylo. Ukrainian principalities under Vitowt were loosing their national character and independence to Polish influences. In 1413 a decision was made to allow only Catholics to occupy important government positions ("Horodlo Privilege"); wide spread discrimination against Orthodox population followed. Nearly all Ukrainians in those days were Orthodox, therefore Ukrainian princes and boyars were helping Svytryhaylo in his fight with Vitowt. After Vitowt died in 1430, Svytryhaylo defended himself from Poles, but by the year 1440 his sphere of influence was reduced to Volynj principality.
There was a period of hostilities between Lithuania and Moscow, when about 1480 several principalities in eastern Ukraine were annexed by Moscow. Also several popular uprising took place. The rebellion under Mukha in 1490, in western Ukraine, was seeking help from neighboring Moldova; uprising under prince Mykhaylo Hlynskiy in 1500 in eastern Ukraine expected help from Moscow and Tatars. However Poland and Lithuania, at that time, were very strong, therefore all uprisings were squashed.
Meanwhile, in the South, marauding Tatar hordes converted large area of the country into wilderness, without any law or order. It was very rich part of Ukraine with productive soil, wild animals and rivers full of fish. It attracted many adventurous people, who although had to fight Tatars there, could be free from suppression by Polish and Lithuanian overlords. They began to organize under hetmans, thus originating Cossack society. To defend themselves from Tatars, they were constructing forts called "sitch" and amalgamated into sort of union, with Zaporizhia, downstream of river Dnipro cascades, as a centre.
In 1552, one of Ukrainian princes, Dmytro Wyshnevetskyi, being among Cossacks, built a castle on island Khortytsya. From there, Cossacks conducted raids on Crimean towns sometimes with help from Moscow. Dmytro wanted to develop Zaporizhia, with help from Lithuania and Moscow, into a powerful fortress against Tatars and Turks. Being unable to achieve this goal, he left Zaporizhia in 1561, became involved in a war in Moldova, was captured and executed by Turks in 1563.
In 1569, by the Union of Lublin, the dynastic link between Poland and Lithuania was transformed into a constitutional union of the two states as the Polish-Lithuanian Commonwealth. Most of Ukraine became part of Poland, settlement of Polish nationals followed, Polish laws and customs became dominant. Most of Ukrainian princes and boyars, except for few notably Ostrozkyis and Wyshnevetskyis -, were replaced by Polish nobles. Peasants lost land ownership and civil rights and gradually became serfs, exploited as manpower in agriculture and forestry, by landowners. Suppression of Orthodox Church retarded development of Ukrainian literature, arts and education; preferential treatment of Catholics inhibited economic and political advancement of Ukrainians.
In spite of that there was a modest revival of Ukrainian culture later in 16th century. Church schools and seminaries were set up, based at first on properties of Ukrainian magnate Hryhoriy Khodkovych and later on holdings of Ostrozkyi princes. Printing industry began, culminating in publication of Bible in print shop ran by Ivan Fedorovych. Trade and church brotherhoods sprang up; they established schools and hospitals and became centers of defense of Orthodox Church and fight for justice and equality.
Such situation also multiplied influx of people to Cossack territory thus increasing Cossacks strength. Tatars were pushed out into Crimea; Cossacks became more daring in their raids on Turkish cities.
Although Ukrainian Cossacks defended not only Ukraine, but also whole eastern Europe from Turks and Tatar hordes, they were causing diplomatic problems for Poland because Turkey used Cossacks as an excuse for wars against Poland. When Cossack leader, Ivan Pidkova, conquered Moldova in 1577, Poles captured and executed him in order to appease the Turks. They tried to control Cossacks by recruiting some of them into Polish military system as, so called, Registered Cossacks, but they could never really tame them.
With decreasing danger from Tatars, Polish nobles and Ukrainian princes loyal to the king, were granted possessions in territory controlled by Cossacks and began to introduce their ,freedom limiting, unpopular laws. Dissatisfied with such treatment Cossacks, under Kryshtof Kosynskyi, rebelled about 1590, and by year 1593 controlled most of eastern Ukraine. After Kosynskyi, Hryhoriy Loboda became Cossack Hetman in 1593.
Another section of Cossacks, numbering about 12000, under Semeryn Nalyvayko, were recruited by Pope and German Kaiser for war against Turks. They conquered Moldova and in 1595 returned to Ukraine to fight against Polish rulers and to defend Orthodox population from Jesuits, who were instigating amalgamation with Catholic Church. In 1596 at a synod of Brest, the Kyivan metropolitan and the majority of bishops signed an act of union with Rome. The Uniate church thus formed recognized supremacy of the pope but retained the Eastern rites and the Slavonic liturgical language.
Also in year 1596 Polish king, Sigismund III Vasa, ordered Field Marshal Stanislav Zholkewski to subjugate Cossack forces. After several months of fighting, Zholkewski surrounded Cossacks, led by Nalyvayko, Loboda and Shaula, at river Solonytsya near Lubny. There were about 6000 Cossack fighters and just as many women and children facing much more superior force. The prolonged siege, lack of food and fodder, internal squabbles (Loboda was killed in one the fights between sections of Cossacks) and intensive cannon fire destroyed defenders' capacity to resist. In order to save their families, Cossacks agreed to Zholkewski's terms to let them go free in exchange for handing over their leaders. However, after surrender, Poles did not keep their word; they attacked and started to massacre defenseless and disoriented Cossacks. Only a section under leadership of Krempskyi broke through and joined with troops of Pidvysotskyi, who were coming to the rescue of besieged Cossacks.
Zholkewski, exhausted by prolonged fighting, decided to abandon the idea to conquer Cossacks. He returned to Poland, where he tortured and executed captured Cossack leaders; most severe punishment was handed to Nalyvayko, who was tortured for about a year prior to a brutal execution.