Succeeding his uncle, Konrad III, he was elected King of the Romans in 1152. He became the ruler of a very much divided and fragmented empire, threatened from within as well as from outside.
Danes, Vikings, Poles and Magyars were invading while rival dukes fought each other instead of the invaders.
A further cause for disharmony was the rivalry with the House of Saxony which had already supplied three German Emperors. To complicate matters even further, there was the fight for supreme power with the church.
In his opinion power was derived directly from God and merely confirmed, not conferred, by the Pope. He needed to secure his power and influence in northern Italy to prevent these Italians from
intriguing with the pope. To achieve this he required peace in Germany and, after his election, made a truce with his kinsman, Heinrich the Lion, by restoring to him Bavaria which had been taken by Konrad III.
In 1154 he toured the Rhineland and the Palatinate, suppressing feuds and executing every peace-breaker he captured. After this he subdued Boleslav of Poland who had tried to become independant from the Empire.
Friedrich Barbarossa, accompanied by a thousand knights, then set out to Italy for his coronation by the English pope, Hadrian IV. First he had to liberate Hadrian IV from the Roman mob. Soon Hadrian IV proclaimed to have "conferred" the imperial crown, after which Friedrich Barbarossa proclaimed throughout the Empire that he had received the imperial crown from God alone.

Fearing the northern Italians might ally themselves with the pope, he appeared in Verona, in June 1158, with an army of fifty thousand men. Then he besieged Milan and, after he had taken the city, made them pay dues to him. However, Hadrian IV died and was replaced by Pope Alexander III, as resolute and intelligent as Friedrich Barbarossa. Friedrich then appointed an antipope, Victor IV, after which Alexander III excommunicated the Emperor and Milan revolted.This time the population was starved out and the city razed to the ground.
Encouraged by Alexander III who had fled to France, the Lombards rebelled against the harsh German administration. Victor IV died and was replaced by another antipope, Paschal III. Friedrich Barbarossa waited three years and then attacked Rome with a large army. Alexander III, returned from France, this time fled to Sicily. Having secured Paschal III in Rome, the plague struck his army and Friedrich Barbarossa was forced to retreat to Germany, crossing the Alps disguised as a servant.
In 1176 he returned to Italy to reassert his authority, but was heavily defeated at Legnano and, in 1177, Friedrich made peace with Alexander III. Nine years later he arranged the marriage of his son to Constance, heiress presumptive of Sicily. Even though in the beginning of his reign he had tried to befried Heinrich the Lion, the latter had refused to assist in Italy and tried to make his Saxon territory semi-independent within the empire. Enemies of Heinrich the Lion complained to Emperor Friedrich who then, when Heinrich the Lion ignored the imperial summons, occupied and divided Saxony in a lightning campaign and banished Heinrich the Lion to France.
During his reign Germany prospered; roads were built and new trade routes were opened. Between 1150 and 1175 the number of German towns doubled, a unified coinage was introduced and literature flourished. Nearly seventy years old, he set out to go on crusade. A large army was collected and went overland towards the Bosphorus while Richard the Lionheart and the French King Philippe went by sea. All went well at first; but when Friedrich Barbarossa reached Byzantine territory, his army was attacked and robbed during the nights. Also, food was scarce while the population had fled, taking everything with them. Then the army sent to attack them by the Byzantine Emperor was defeated, after which they received Byzantine support.
They were a year on the road before arriving in Asia Minor where they were attacked by hostile horsemen. His army marched through the mountains where again there was no food; yet half-starved, they continued, now reduced to six hundred knights who, become delirious, saw visions. Nevertheless they still attacked and conquered Iconium which had been defended by a much stronger army. Later, with food aplenty, they recovered to continue their way to the Holy Land. There had been one prophecy: that he would die by drowning; yet another, that he would win his empire like a fox, preserve it like a lion, but die like a dog.
They were near Seleucia in the intense heat of June when they crossed a fast flowing river. Resting in the burning sun, he had some food, then decided to bathe in the river. Against advice, he went in
and his men saw him dissappear. When his body was found much later, his knights decided to return home.
However, according to myth, he never died but is simply asleep, one day to return and save Germany from its enemies.

Source: Leo van de Pas