King Casimir III of Poland initiates campaigns to take Galicia-Volhynia upon the death of Bolesław Jerzy II of Mazovia in the mid-fourteenth century; these occur from 1340 to 1366.

The heartland of Rus', including Kiev, meanwhile becomes the territory of the Grand Duchy of Lithuania, ruled by Gediminas and his successors, after the semi-legendary Battle on the Irpein River.

Following the 1386 Union of Krewo, a dynastic union between Poland and Lithuania, much of what will become northern Ukraine is ruled by the increasingly Slavicised local Lithuanian nobles as part of the Grand Duchy of Lithuania.

The so-called Galicia–Volhynia Wars end by 1392.

Polish colonizers of depopulated lands in northern and central Ukraine soon found or re-found many towns.

Mindaugas of Lithuania had been murdered in 1263, and in the ensuing chaos, the lands of the Grand Duchy are in disarray, with both local and foreign rulers struggling for power.

Shvarn gives his support to Vaišvilkas, one of Mindaugas' sons and his brother-in-law.

Together, they manage to depose Treniota and expel Daumantas all the way to Pskov.

Boleslaw mounts a counteroffensive against Shvarn and his uncle Vasilko Romanovich and defeats the former on June 19, 1266 at Wrota.

This weakens Shvarn's position in his own domain.

Shvarn becomes the new Grand Duke in 1267 after Vaišvilkas returns to monastic life.

No details are known about Shvarn's rule over Lithuania and he probably did not gain a strong foothold in that country.

However, he is apparently fairly successful in expanding his borders.

Volhynia had passed into the control of the Lithuanian prince Liubartas after the extinction of the Rurikid dynasty in Galicia–Volhynia in 1323, while the boyars had taken control over Galicia.

They had invited the Polish Prince Boleslaw Yuri II, a grandson of Yuri I, prince of Galicia, to assume the Galician throne.

Boleslaw was born between 1305 and 1310 to Trojden I of Masovia, Duke of Czersk, from the Piast dynasty, and Maria, daughter of Yuri I.

Since his father was still a ruler of the family's Mazovian lands, in 1323 Boleslaw, renamed Jerzy, had become Prince of Galicia.

He also received the Duchy of Belz after the childless death of Andrew of Galicia.

He had in 1331 married the daughter of Grand Duke of Lithuania Gediminas and sister of Aldona of Lithuania, wife of Casimir III of Poland.

Boleslaw Jerzy  in a treaty of 1338 had offered Casimir III of Poland the succession to the throne of Galicia-Volhynia.

Duke Boleslaw is supported by the many German townspeople already living here.

Boleslaw has converted to Orthodoxy and assumed the name Yuri II.

Nevertheless, the boyars, suspecting him of harboring Catholic feelings, poison him in 1340 and elect one of their own, Dmitry Detko, to lead the Galician state.

The kingdom of Galicia will be gradually annexed by the kingdom of Poland between 1340 and 1366, during the reign of Casimir III.

Casimir III has annexed the Halych–Volodymyr area of Rus' in the course of Poland’s expansion to the east through a series of military campaigns between 1340 and 1366.

The town of Lviv here attracts newcomers of several nationalities, is granted municipal rights in 1356, and thus begins its career as Lwów, the main Polish center amidst a Rus' Orthodox population.

...Lwów, and ...

A Lithuanian garrison still resists the Cossacks' siege in Stary Bykhov, when Khmelnitsky and Buturlin are already active in Galicia.

They attack the Polish city of Lwów in September.

Russian and Cossack forces have meanwhile occupied the east of the Polish-Lithuanian Commonwealth as far as Lublin, with only Lwow (Lviv, Lemberg) remaining under Polish-Lithuanian control. 

John II Casimir, with the Lwów Oath of April 1, 1656, had crowned Our Lady of Częstochowa (the Black Madonna) as Queen and Protector of Poland in the cathedral of Lwów after the miraculous saving of the monastery of Jasna Góra during The Deluge, an event which has actually changed the course of the war.

He has promised to lift the burdens inflicted on the peasantry if he regains control.