Louis's brother Andrew was murdered in Aversa on 18 September 1345.
Louis and his mother accused Queen Joanna I, Prince Robert of Taranto, Duke Charles of Durazzo, and other members of the Neapolitan branches of the Capetian House of Anjou of plotting against Andrew.
In his letter of 15 January 1346 to Pope Clement VI, Louis demanded that the pope dethrone the "husband-killer" queen in favor of Charles Martel, her infant son by Andrew.
Louis also laid claim to the regency of the kingdom during the minority of his nephew, referring to his patrilinear descent from the first-born son of Robert the Wise's father, Charles II of Naples.
He even promised to increase the amount of yearly tribute that the kings of Naples would pay to the Holy See.
After the pope failed to fully investigate Andrew's murder, Louis decided to invade southern Italy.
In preparation for the invasion, he sent his envoys to Ancona and other Italian towns before summer 1346.

While his envoys negotiated in Italy, Louis marched to Dalmatia to relieve Zadar, but the Venetians bribed his commanders.
When the citizens broke out and attacked the besiegers on 1 July, the royal army failed to intervene, and the Venetians overcame the defenders outside the walls of the town.
Louis withdrew but refused to renounce Dalmatia, although the Venetians offered to pay 320,000 golden florins as compensation.
Lacking military support from Louis, however, Zadar surrendered to the Venetians on 21 December 1346.

Louis sent small expeditions one after one to Italy at the beginning of his war against Joanna, because he did not want to harass the Italians who had suffered from a famine the previous year.
His first troops departed under the command of Nicholas Vásári, Bishop of Nyitra (now Nitra in Slovakia), on 24 April 1347.
Louis also hired German mercenaries.
He departed from Visegrád on 11 November.
After marching through Udine, Verona, Modena, Bologna, Urbino, and Perugia, he entered the Kingdom of Naples on 24 December near L'Aquila, which had yielded to him.

Queen Joanna remarried, wedding a cousin, Louis of Taranto, and fled for Marseille on 11 January 1348.
Their other relatives, Robert of Taranto and Charles of Durazzo, visited Louis in Aversa to yield to him.
Louis received them amicably and convinced them to persuade their brothers, Philip of Taranto and Louis of Durazzo, to join them.
After their arrival, King Louis's "smile was replaced by the harshest expression as he unveiled with terrible words the true feelings he had for the princes and that he had kept hidden until then", according to the contemporaneous Domenico da Gravina.
He repeated his former accusations, blamed his kinsmen for his brother's murder, and had them captured on 22 January.
The next day Charles of Durazzo –  the husband of Joanna I's sister, Mary – was beheaded upon Louis's orders.
The other princes were kept captive and sent to Hungary, together with Louis's infant nephew, Charles Martel.

Louis marched to Naples in February.
The citizens offered him a ceremonious entry, but he refused, threatening to let his soldiers sack the town if they did not raise the taxes.
He adopted the traditional titles of the kings of Naples – "King of Sicily and Jerusalem, Duke of Apulia and Prince of Capua" – and administered the kingdom from the Castel Nuovo, garrisoning his mercenaries in the most important forts.
He used unusually brutal methods of investigation to capture all accomplices in the death of his brother, according to Domenico da Gravina.
Most local noble families (including the Balzos and the Sanseverinos) refused to cooperate with him.
The pope refused to confirm Louis's rule in Naples, which would have united two powerful kingdoms under Louis's rule.
The pope and the cardinals declared Queen Joanna innocent of her husband's murder at a formal meeting of the College of Cardinals.

The arrival of the Black Death forced Louis to leave Italy in May. He made Ulrich Wolfhardt governor of Naples, but his mercenaries did not hinder Joanna I and her husband from returning in September.
Louis, who had sign
Louis's brother Andrew was murdered in Aversa on 18 September 1345. Louis and his mother accused Queen Joanna I, Prince Robert of Taranto, Duke Charles of Durazzo, and other members of the Neapolitan branches of the Capetian House of Anjou of plotting against Andrew. In his letter of 15 January 1346 to Pope Clement VI, Louis demanded that the pope dethrone the "husband-killer" queen in favor of Charles Martel, her infant son by Andrew. Louis also laid claim to the regency of the kingdom during the minority of his nephew, referring to his patrilinear descent from the first-born son of Robert the Wise's father, Charles II of Naples. He even promised to increase the amount of yearly tribute that the kings of Naples would pay to the Holy See. After the pope failed to fully investigate Andrew's murder, Louis decided to invade southern Italy. In preparation for the invasion, he sent his envoys to Ancona and other Italian towns before summer 1346.
While his envoys negotiated in Italy, Louis marched to Dalmatia to relieve Zadar, but the Venetians bribed his commanders. When the citizens broke out and attacked the besiegers on 1 July, the royal army failed to intervene, and the Venetians overcame the defenders outside the walls of the town. Louis withdrew but refused to renounce Dalmatia, although the Venetians offered to pay 320,000 golden florins as compensation. Lacking military support from Louis, however, Zadar surrendered to the Venetians on 21 December 1346.
Louis sent small expeditions one after one to Italy at the beginning of his war against Joanna, because he did not want to harass the Italians who had suffered from a famine the previous year. His first troops departed under the command of Nicholas Vásári, Bishop of Nyitra (now Nitra in Slovakia), on 24 April 1347. Louis also hired German mercenaries. He departed from Visegrád on 11 November. After marching through Udine, Verona, Modena, Bologna, Urbino, and Perugia, he entered the Kingdom of Naples on 24 December near L'Aquila, which had yielded to him.
Queen Joanna remarried, wedding a cousin, Louis of Taranto, and fled for Marseille on 11 January 1348. Their other relatives, Robert of Taranto and Charles of Durazzo, visited Louis in Aversa to yield to him. Louis received them amicably and convinced them to persuade their brothers, Philip of Taranto and Louis of Durazzo, to join them. After their arrival, King Louis's "smile was replaced by the harshest expression as he unveiled with terrible words the true feelings he had for the princes and that he had kept hidden until then", according to the contemporaneous Domenico da Gravina. He repeated his former accusations, blamed his kinsmen for his brother's murder, and had them captured on 22 January. The next day Charles of Durazzo –  the husband of Joanna I's sister, Mary – was beheaded upon Louis's orders. The other princes were kept captive and sent to Hungary, together with Louis's infant nephew, Charles Martel.
Louis marched to Naples in February. The citizens offered him a ceremonious entry, but he refused, threatening to let his soldiers sack the town if they did not raise the taxes. He adopted the traditional titles of the kings of Naples – "King of Sicily and Jerusalem, Duke of Apulia and Prince of Capua" – and administered the kingdom from the Castel Nuovo, garrisoning his mercenaries in the most important forts. He used unusually brutal methods of investigation to capture all accomplices in the death of his brother, according to Domenico da Gravina. Most local noble families (including the Balzos and the Sanseverinos) refused to cooperate with him. The pope refused to confirm Louis's rule in Naples, which would have united two powerful kingdoms under Louis's rule. The pope and the cardinals declared Queen Joanna innocent of her husband's murder at a formal meeting of the College of Cardinals.
The arrival of the Black Death forced Louis to leave Italy in May. He made Ulrich Wolfhardt governor of Naples, but his mercenaries did not hinder Joanna I and her husband from returning in September. Louis, who had sign
Louis I of Hungary