Frederick Barbarossa (December 1122 10 June 1190), also known as Frederick I (German: Friedrich I, Italian: Federico I), was the Holy Roman Emperor from 1155 until his death 35 years later. He was elected King of Germany at Frankfurt on 4 March 1152 and crowned in Aachen on 9 March 1152. He was crowned King of Italy on 24 April 1155 in Pavia and emperor by Pope Adrian IV on 18 June 1155 in Rome. Two years later, the term sacrum ("holy") first appeared in a document in connection with his empire. He was later formally crowned King of Burgundy, at Arles on 30 June 1178. He was named Barbarossa by the northern Italian cities which he attempted to rule: Barbarossa means "red beard" in Italian; in German, he was known as Kaiser Rotbart, which means "Emperor Redbeard" in English. The prevalence of the Italian nickname, even in later German usage, reflects the centrality of the Italian campaigns to his career.

Before his imperial election, Frederick was by inheritance Duke of Swabia (11471152, as Frederick III). He was the son of Duke Frederick II of the Hohenstaufen dynasty and Judith, daughter of Henry IX, Duke of Bavaria, from the rival House of Welf. Frederick, therefore, descended from the two leading families in Germany, making him an acceptable choice for the Empire's prince-electors.

Historians consider him among the Holy Roman Empire's greatest medieval emperors. He combined qualities that made him appear almost superhuman to his contemporaries: his longevity, his ambition, his extraordinary skills at organization, his battlefield acumen and his political perspicacity. His contributions to Central European society and culture include the reestablishment of the Corpus Juris Civilis, or the Roman rule of law, which counterbalanced the papal power that dominated the German states since the conclusion of the Investiture Controversy.

Due to his popularity and notoriety, in the nineteenth and early twentieth centuries, he was used as a political symbol by many movements and regimes: the Risorgimento, the Wilhelmine government in Germany (especially under Emperor Wilhelm I) and the National Socialist movement, resulting in both golden and dark legends. Modern researchers, while exploring the legacy of Frederick, attempt to uncover the legends and reconstruct the true historical figurethese efforts result in new perspectives on both the emperor as a person and social developments associated with him.

Frederick died in 1190 in Asia Minor while leading an army in the Third Crusade.

The Third Crusade (1189–1192) was an attempt by three European monarchs of Western Christianity (Philip II of France, Richard I of England and Frederick I, Holy Roman Emperor) to reconquer the Holy Land following the capture of Jerusalem by the Ayyubid sultan Saladin in 1187. For this reason, the Third Crusade is also known as the Kings' Crusade.It was partially successful, recapturing the important cities of Acre and Jaffa, and reversing most of Saladin's conquests, but it failed to recapture Jerusalem, which was the major aim of the Crusade and its religious focus.

After the failure of the Second Crusade of 1147–1149, the Zengid dynasty controlled a unified Syria and engaged in a conflict with the Fatimid rulers of Egypt. Saladin ultimately brought both the Egyptian and Syrian forces under his own control, and employed them to reduce the Crusader states and to recapture Jerusalem in 1187. Spurred by religious zeal, King Henry II of England and King Philip II of France (known as "Philip Augustus") ended their conflict with each other to lead a new crusade. The death of Henry (6 July 1189), however, meant the English contingent came under the command of his successor, King Richard I of England.

The elderly German Emperor Frederick Barbarossa also responded to the call to arms, leading a massive army across the Balkans and Anatolia. He achieved some victories against the Seljuk Sultanate of Rûm, but he drowned in a river on 10 June 1190 before reaching the Holy Land. His death caused tremendous grief among the German Crusaders, and most of his troops returned home.

After the Crusaders had driven the Muslims from Acre, Philip—in company with Frederick's successor in command of the German crusaders, Leopold V, Duke of Austria—left the Holy Land in August 1191. Following a major victory by the Crusaders at the Battle of Arsuf, most of the coastline of the Levant was returned to Christian control. On 2 September 1192 Richard and Saladin finalized the Treaty of Jaffa, which recognised Muslim control over Jerusalem but allowed unarmed Christian pilgrims and merchants to visit the city. Richard departed the Holy Land on 9 October 1192. The successes of the Third Crusade allowed Westerners to maintain considerable states in Cyprus and on the Syrian coast.

The failure to re-capture Jerusalem inspired the subsequent Fourth Crusade of 1202–1204, but Europeans would only regain the city—and only briefly—in the Sixth Crusade in 1229.