Louis I of Hungary
Union with Poland and reforms (1370–1377)

Casimir III of Poland died on 5 November 1370. Louis arrived after his uncle's funeral and ordered the erection of a splendid Gothic marble monument to the deceased king. He was crowned king of Poland in the Kraków Cathedral on 17 November. Casimir III had willed his patrimony – including the duchies of Sieradz, Łęczyca and Dobrzyń – to his grandson, Casimir IV, Duke of Pomerania. However, the Polish prelates and lords were opposed to the disintegration of Poland and Casimir III's testament was declared void. Louis visited Gniezno and made his Polish mother, Elizabeth, regent before returning to Hungary in December. His uncle's two surviving daughters (Anna and Jadwiga) accompanied him, and the Polish Crown Jewels were transferred to Buda, which raised discontent among Louis's new subjects. Louis's wife gave birth to a daughter, Catherine, in 1370, seventeen years after their marriage; a second daughter, Mary, was born in 1371. Thereafter Louis's made several attempts to safeguard his daughters' right to succeed him.

During a war between Emperor Charles IV and Stephen II, Duke of Bavaria, Louis intervened on the duke's behalf and the Hungarian army invaded Moravia. After the duke and the emperor signed a peace treaty, Louis and the emperor agreed upon the betrothal of their children early the next year. The Ottomans annihilated the Serbian armies in the Battle of Marica on 26 September 1371. Lazar Hrebeljanović, one of the Serbian lords, swore loyalty to Louis. Pope Gregory XI urged Louis to resist the Ottomans but also pleaded with him to send reinforcements to Italy to fight against Bernabò Visconti. A war broke out between the Republic of Venice and Francesco I da Carrara, Lord of Padova, who was an ally of Louis, in the summer of 1372. Louis sent reinforcements to Italy to assist Francesco da Carrara. The Venetians defeated the Hungarian troops at Treviso and captured its commander, Nicholas Lackfi, forcing Louis I to sign a peace treaty on 23 September 1373.

Louis and the representatives of the Polish nobility started negotiations of Louis's succession in Poland in the autumn of 1373. After a year of negotiations, he issued the so-called Privilege of Koszyce on 17 September 1374, reducing the tax that Polish noblemen paid to the king by about 84% and promising a remuneration to noblemen who participated in foreign military campaigns. In exchange, the Polish lords confirmed the right of Louis's daughters to inherit Poland.

Louis invaded Wallachia in May 1375, because the new prince of Wallachia, Radu I, had formed an alliance with the Bulgarian ruler, Ivan Shishman, and the Ottoman Sultan Murad I. The Hungarian army routed the united forces of the Wallachians and their allies, and Louis occupied the Banate of Severin, but Radu I did not yield. During the summer, Wallachian troops stormed into Transylvania and Ottomans pillaged the Banat.

From the middle of the 1370s, the Lackfis' influence diminished and new favorites emerged in the royal court. James Szepesi was appointed judge royal in 1373, and Nicholas Garay became the palatine in 1375. The organization of central government was also modified to create a more centralized power structure. Louis's "secret seal", that he had always taken with him during his wars and journeys, was declared authentic, and Louis entrusted it to the secret chancellor who was always to accompany him. A new high official, the Lord Chancellor were authorized to use the great seal in the king's name in 1376 or 1377. Demetrius, Bishop of Zagreb, who was of humble origin, was the first to hold this new office. The Lord Chancellor became the head of a new central court of justice, called the court of "the king's special presence" in 1377. From around the same time, the royal free towns delegated jurors to assist the master of the treasury, who headed the court of appeal for the towns. A new official, the treasurer, took over the financial duties of the master of the treasury.

The Lithuanians made raids in Halych, Lodomeria, and Poland, almost reaching Kraków in November 1376. A riot broke out in Kraków against the unpopular queen mother, Elizabeth, on 6 December. The rioters slaughtered about 160 servants of the queen-mother, forcing her to flee to Hungary. Taking advantage of the situation, Władysław the White, Duke of Gniewkowo, who was a male member of the royal Piast dynasty, announced his claim to the Polish crown. However, Louis's partisans defeated the pretender, and Louis made him abbot of the Pannonhalma Archabbey in Hungary. Louis appointed Vladislaus II of Opole his governor in Poland. In summer 1377, Louis invaded the territories held by the Lithuanian prince, George, in Lodomeria. His Polish troops soon captured Chełm, while Louis seized George's seat, Belz, after besieging it for seven weeks. He incorporated the occupied territories in Lodomeria, together with Galicia, into the Kingdom of Hungary. Three Lithuanian princes – Fedor, Prince of Ratno, and two princes of Podolia, Alexander and Boris – accepted Louis's suzerainty.
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