structural:reformatting, linguistic:paraphrasing, quality:correction, structural:scholar_edition
War of succession (1205–1245)

In 1205, Roman's alliance with the Poles broke down, leading to a conflict with Leszek the White and Konrad of Masovia. Roman was subsequently killed by Polish forces in the Battle of Zawichost (1205), triggering a war of succession, while his dominion entered a period of rebellion and chaos that lasted almost 40 years. In this time, the Galician boyars made efforts to prevent the establishment of a hereditary princely dynasty, especially by Roman's son Daniel, and instead put all sorts of puppets on the throne which they could easily control. Thus weakened by war between Galician boyars and some appanage princes, Galicia–Volhynia also became an arena of rivalry between Poland and Hungary, which intervened in the region several times. Roman's successors would mostly use Halych (Galicia) as the designation of their combined kingdom. King Andrew II of Hungary styled himself rex Galiciæ et Lodomeriæ, Latin for "king of Galicia and Vladimir ", a title that was later adopted by the House of Habsburg.

After Roman's death, the Galician boyars first drove Roman's widow Anna-Euphrosyne and two sons Daniel and Vasylko from the region. From 1206 to 1212, the Principality of Galicia was controlled by the three sons of the Novgorod-Seversk prince Igor Svyatoslavich: Vladimir III Igorevich, Svyatoslav III Igorevich, and Roman II Igorevich. They were defeated by Galician boyars, and the boyar Volodyslav Kormylchych assumed the throne of Galicia in 1213 or 1214, the only non-Rurikid ever to rule any of the Rus' principalities. After he was removed, a compromise agreement was concluded in 1214 between Hungary and Poland, who partitioned the Galician lands. The throne of Galicia–Volhynia was given to Andrew's son, Coloman of Lodomeria, who had married Leszek the White's daughter, Salomea.

In 1221, Mstislav Mstislavich, son of Mstislav Rostislavich (descendant of the princes of Novgorod), liberated Galicia–Volhynia from the Hungarians and Poles. During Mstislav's 1221–1228 reign, the Galician and Volhynian armies participated in the Battle of the Kalka River (1223) against the Mongols, but in 1228 the boyars expelled him and transferred the Principality of Galicia to the king of Hungary. It was Daniel of Galicia, son of Roman, who formed a real union of Volhynia and Galicia. Daniel first established himself in Volhynia. After failing to retake his father's other throne in 1230–1232 and 1233–1235, Daniel succeeded upon his third attempt and conquered Galicia in 1238, reunited Galician and Volhynia, and ruled for a quarter century. In March 1238, he defeated the Teutonic Knights of the Order of Dobrzyń in the Battle of Dorohychyn. Daniel captured Kiev in 1239, just before the Mongols besieged, conquered and sacked the city in late 1240. On 17 August 1245, Daniel and his brother Vasylko defeated the Polish and Hungarian forces (weakened by the first Mongol invasion of Poland and the first Mongol invasion of Hungary in early 1241) in the Battle of Yaroslav (Jarosław), taking full control of Galicia–Volhynia. The brothers also crushed their ally Rostislav Mikhailovich, son of the prince of Chernigov.