Casimir III of Poland urged Louis to intervene in his war with the Lithuanians who had occupied Brest, Volodymyr-Volynskyi, and other important towns in Halych and Lodomeria in the previous years.
The two monarchs agreed that Halych and Lodomeria would be integrated into the Kingdom of Hungary after Casimir's death.
Casimir also authorized Louis to redeem the two realms for 100,000 florins if Casimir fathered a son.
Louis led his army to Cracow in June 1351.
Because Casimir fell ill, Louis became the sole commander of the united Polish and Hungarian army.
He invaded the lands of the Lithuanian prince, Kęstutis, in July.
Kęstutis seemingly accepted Louis's suzerainty on 15 August and agreed to be baptised, along with his brothers, in Buda.
However, Kęstutis did nothing to fulfill his promises after Polish and Hungarian troops were withdrawn.
In an attempt to capture Kęstutis, Louis returned, but he could not defeat the Lithuanians, who even killed one of his allies, Boleslaus III of Płock, in battle.
Louis returned to Buda before 13 September.
A papal legate visited Louis to persuade him to wage war against Stefan Dušan, Emperor of the Serbs, who had forced his Roman Catholic subjects to be re-baptised and join the Serbian Orthodox Church.

To deal with the grievances of the Hungarian noblemen, Louis held a Diet in late 1351.
He confirmed all but one of the provisions of the Golden Bull of 1222, declaring that all noblemen enjoyed the same liberties in his realms.
He rejected only the provision that authorized noblemen who died without a son to freely bequeath their estates.
Instead, he introduced an entail system, prescribing that the estates of a nobleman who had no male descendants passed to his kinsmen, or if there were no male relatives to the Crown, upon his death.
At the same Diet, Louis ordered that all landowners were to collect the "ninth", that is one tenth of specified agricultural products, from the peasants who held plots on their estates.
On the other hand, he confirmed the right of all peasants to freely move to another landowner's estates.

The "general accord" between Louis and the royal couple of Naples "was accepted by both sides" during 1351, according to the contemporaneous Niccolò Acciaioli.
Joanna I and her husband returned to the Kingdom of Naples and Louis's troops were withdrawn.
Louis even renounced the ransom that Joanna I had promised to pay for the liberation of the imprisoned Neapolitan princes, stating that he had not gone to "war for greed, but to avenge the death of his brother".
Louis continued to use the titles of his grandfather, Charles Martel of Anjou (the firstborn son of Charles II of Naples), styling himself as "Prince of Salerno and lord of Monte Sant'Angelo".

Casimir III laid siege to Belz and Louis joined his uncle in March 1352.
During the siege, which ended without the surrender of the fort, Louis was heavily injured in his head.
Algirdas, Grand Duke of Lithuania, hired Tatar mercenaries who stormed into Podolia, Louis returned to Hungary because he feared a Tatar invasion of Transylvania.
Pope Clement proclaimed a crusade against the Lithuanians and the Tatars in May, authorizing Louis to collect a tithe from Church revenues during the next four years.
The pope stated that he had never "granted a tenth of such duration", emphasizing the link between his magnanimity and the release of the imprisoned Neapolitan princes.
The pope also authorized Louis to seize the pagans' and schismatics' lands bordering on his kingdom.

Although Louis signed an alliance with the Republic of Genoa in October 1352, he did not intervene in the Genoese–Venetian War, because his truce of 1349 with Venice was still in force.
Louis married Elizabeth of Bosnia, who was the daughter of his vassal, Stephen II, in 1353.
Historian Gyula Kristó says that this marriage showed Louis's renewed interest in the affairs of the Balkan Peninsula.
While he was hunting in Zólyom County (now in Slovakia) in late November 1353, a brown bear attacked him, inflicting 24 wounds on his legs.
Louis's life was saved by a knig
Casimir III of Poland urged Louis to intervene in his war with the Lithuanians who had occupied Brest, Volodymyr-Volynskyi, and other important towns in Halych and Lodomeria in the previous years. The two monarchs agreed that Halych and Lodomeria would be integrated into the Kingdom of Hungary after Casimir's death. Casimir also authorized Louis to redeem the two realms for 100,000 florins if Casimir fathered a son. Louis led his army to Cracow in June 1351. Because Casimir fell ill, Louis became the sole commander of the united Polish and Hungarian army. He invaded the lands of the Lithuanian prince, Kęstutis, in July. Kęstutis seemingly accepted Louis's suzerainty on 15 August and agreed to be baptised, along with his brothers, in Buda. However, Kęstutis did nothing to fulfill his promises after Polish and Hungarian troops were withdrawn. In an attempt to capture Kęstutis, Louis returned, but he could not defeat the Lithuanians, who even killed one of his allies, Boleslaus III of Płock, in battle. Louis returned to Buda before 13 September. A papal legate visited Louis to persuade him to wage war against Stefan Dušan, Emperor of the Serbs, who had forced his Roman Catholic subjects to be re-baptised and join the Serbian Orthodox Church.
To deal with the grievances of the Hungarian noblemen, Louis held a Diet in late 1351. He confirmed all but one of the provisions of the Golden Bull of 1222, declaring that all noblemen enjoyed the same liberties in his realms. He rejected only the provision that authorized noblemen who died without a son to freely bequeath their estates. Instead, he introduced an entail system, prescribing that the estates of a nobleman who had no male descendants passed to his kinsmen, or if there were no male relatives to the Crown, upon his death. At the same Diet, Louis ordered that all landowners were to collect the "ninth", that is one tenth of specified agricultural products, from the peasants who held plots on their estates. On the other hand, he confirmed the right of all peasants to freely move to another landowner's estates.
The "general accord" between Louis and the royal couple of Naples "was accepted by both sides" during 1351, according to the contemporaneous Niccolò Acciaioli. Joanna I and her husband returned to the Kingdom of Naples and Louis's troops were withdrawn. Louis even renounced the ransom that Joanna I had promised to pay for the liberation of the imprisoned Neapolitan princes, stating that he had not gone to "war for greed, but to avenge the death of his brother". Louis continued to use the titles of his grandfather, Charles Martel of Anjou (the firstborn son of Charles II of Naples), styling himself as "Prince of Salerno and lord of Monte Sant'Angelo".
Casimir III laid siege to Belz and Louis joined his uncle in March 1352. During the siege, which ended without the surrender of the fort, Louis was heavily injured in his head. Algirdas, Grand Duke of Lithuania, hired Tatar mercenaries who stormed into Podolia, Louis returned to Hungary because he feared a Tatar invasion of Transylvania. Pope Clement proclaimed a crusade against the Lithuanians and the Tatars in May, authorizing Louis to collect a tithe from Church revenues during the next four years. The pope stated that he had never "granted a tenth of such duration", emphasizing the link between his magnanimity and the release of the imprisoned Neapolitan princes. The pope also authorized Louis to seize the pagans' and schismatics' lands bordering on his kingdom.
Although Louis signed an alliance with the Republic of Genoa in October 1352, he did not intervene in the Genoese–Venetian War, because his truce of 1349 with Venice was still in force. Louis married Elizabeth of Bosnia, who was the daughter of his vassal, Stephen II, in 1353. Historian Gyula Kristó says that this marriage showed Louis's renewed interest in the affairs of the Balkan Peninsula. While he was hunting in Zólyom County (now in Slovakia) in late November 1353, a brown bear attacked him, inflicting 24 wounds on his legs. Louis's life was saved by a knig
Louis I of Hungary