thumb|Louis II, from the "Emperor Windows" of [[Strasbourg Cathedral#Nave stained glass - The Emperor Windows|Strasbourg Cathedral]]
Louis II (825 – 12 August 875), sometimes called the Younger, was the king of Italy and emperor of the Carolingian Empire 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 the Eastern Roman Empire. He was called imperator Italiae ("emperor of Italy") in West Francia while the Byzantines called him Basileus Phrangias ("Emperor of Francia").
The chronicler Andreas of Bergamo, who is the main source for Louis's activities in southern Italy, notes that "after his death a great tribulation came to Italy."
Childhood
thumb|220px|Division of the Carolingian Empire under the [[Treaty of Prüm (855)]]
Louis was born in 825, the eldest son of the junior emperor Lothair I and his wife 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 grew up in his grandfather's court and probably developed a warm affection for the emperor who, in 839, designated his grandson as King of Italy, and let Louis take up his residence in that country.
Louis the Pious died the next year, and his empire was partitioned between his sons: Louis' father, Lothair, and Louis' uncle, Louis the German, as well as their younger half-brother, Charles the Bald. Under his father's rule, he was crowned king and co-emperor to the middle-aged Emperor Lothair I at Rome by Pope Sergius II on 15 June 844. This ceremony mirrors the crowning of Lothair I by his father, a tradition started by Charlemagne and his son Louis the Pious, who were, respectively, great-grandfather and grandfather of Louis II.
Joint emperor
Louis II immediately claimed the rights of an emperor in the city, a claim which 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, while Salerno was given to Siconulf as an independent principality. Radelchis, now pacified, had no need of his Aghlabid 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.
Sole emperor
thumb|220px|Expansion of Louis' domains, after the cessions from [[Lothar II in 859 and the death of his brother Charles of Provence in 863]]
Carolingian interventions
thumb|220px|Louis II is left without prize after the death of his brother [[Lothar II in 869]]
The division of Lothair's domains under the Treaty of Prüm, by which Louis obtained no territory outside Italy, aroused his discontent, and in 857 he allied himself with Louis the German against his own brother Lothair II, and King Charles the Bald. But after Louis had secured the election of Pope Nicholas I in 858, he 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 routed these invaders, but could not follow up his successes owing to the lack of a fleet. So in 869 he made alliance with the eastern emperor, Basil I, who sent him ships to assist in the capture of Bari, capital of the Emirate of Bari, which succumbed in 871.
thumb|upright|Louis' gravestone in Milan.
Louis won further successes against the Aghlabids, relieving the siege of Salerno and driving them from Capua, but the emperor's attempts to punish Adelchis were not very successful. Returning to northern Italy, 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.