Louis II, sometimes called the Younger, was the King of Italy and Roman Emperor from 844, co-ruling with his father Lothair I until 855, after which he ruled alone. Louis's usual title was imperator augustus ("august emperor"), but he used imperator Romanorum ("emperor of the Romans") after his conquest of Bari in 871, which led to poor relations with Byzantium. He was called imperator Italiae ("emperor of Italy") in West Francia while the Byzantines called him Basileus Phrangias ("Emperor of the Franks"). The chronicler Andreas Bergomatis, who is the main source for Louis's activities in southern Italy, notes that "after his death a great tribulation came to Italy".
Louis was born in 825; the eldest son of the Junior Emperor Lothair I and Ermengarde of Tours. His father was the son of the reigning Emperor, Louis the Pious. Little is known about his early life, except that he passed the time in his grandfather's court and probably developed a warm affection with the old king, who in 839, designated his 14-year-old grandson as King of Italy, and let Louis (Ludovico) take up his residence in that country. His grandfather, the elderly Emperor Louis I, died the next year and his empire was partitioned between his sons, Louis' father, Lothair, and uncle, Louis the German. Under his father's rule, he was crowned king and co-emperor to the middle aged Emperor Lothair at Rome by Pope Sergius II on 15 June 844. This ceremony mirrors the crowning of Lothair by his father, a tradition started by Charlemagne and his son Louis the Pious, who were, respectively, great grandfather and grandfather of Louis II.
Louis II immediately claimed the rights of an emperor in the city, which claim was decisively rejected; but in 850 he was crowned joint emperor at Rome by Pope Leo IV, and soon afterwards, in 851, married Engelberga and undertook the independent government of Italy. He marched into the south of Italy in the year of his imperial coronation and compelled the rival dukes of Benevento, Radelchis I and Siconulf, to make peace. His mediation split the Lombard duchy and gave Radelchis his share with Benevento as his capital and gave Salerno as a principality independent to Siconulf. Radelchis, now pacified, had no need of his Saracen mercenaries and happily betrayed them to the emperor. Louis fell on them and they were massacred. He then quashed some accusations against Pope Leo and held a Diet at Pavia. He confirmed the usurping regent Peter as prince of Salerno in December 853, displacing the dynasty he had installed there three years earlier. On the death of his father in September 855, he became sole emperor.
The division of Lothair's dominions, by which he obtained no territory outside Italy, aroused his discontent, and in 857 he allied himself with Louis the German against his own brother Lothair, King of Lotharingia, and King Charles the Bald. But after Louis had secured the election of Pope Nicholas I in 858, he became reconciled with his brother, and received some lands south of the Jura mountains in return for assistance given to Lothair in his efforts to obtain a divorce from his wife, Teutberga.
He died near Ghedi, in what is now the province of Brescia, on 12 August 875, having named as his successor in Italy his cousin Carloman, son of Louis the German. Louis was buried in the Basilica of Sant'Ambrogio in Milan.