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.

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.