The Holy Roman Empire was a historical state in central Europe, claiming to continue the legacy of the old Western Roman Empire. It was a complex, multi-ethnic union of territories whose internal politics were dominated by the power struggle between the Emperor, the Christian Pope, and the rulers of the various constituent states.
The Empire has its origins in the conquests of Charles I, Duke of the Franks, commonly known as Charlemagne. During the second half of the 8th century he attacked and subjugated many of the neighbouring tribes and nations, eventually growing strong enough to challenge the Gothic hegemony over Western Europe. In 798 he led a daring raid deep into Hispania, capturing the imperial regalia of the Western Empire and eventually using it to have himself crowned Holy Roman Emperor by the Pope.
Initially Lyonesse was also a part of the HRE, but it became permanently separated after the extinction of the Carolingian dynasty. In the eastern, mostly German-speaking part, a system of electing future emperors was developed over time by the leaders of several of the most powerful subject states.
From the very beginning the Pope was simultaneously the overlord of the HRE, in his role as the leader of Christendom, and one of its vassals as King of the Papal Kingdom. This led to a unique form of power struggle between the Emperor, whose base of support lay in the German lands, and the Pope who was based in central Italy. Northern Italy, which was caught between the two, soon began a process of disintegration into many semi-independent fiefs and city-states, which accelerated after the 12th century Roman conquest of the south.
In 1096 the HRE took part in the Great Crusade called by Pope Urban II. However, after the Crusade's defeat at the Battle of Paphos, most of its leaders abandoned the quest and returned home. In 1104 Romania invaded and conquered the Papal Kingdom in retaliation, forcing future Popes to live as guests of the Emperor in Mainz.
For the next few centuries Holy Roman Emperors were able to rule mostly unchallenged by the ecclesiastical authorities.
Islamification and decentralisation
From the 12th century onwards many of the HRE's neighbours began to convert to Islam. Italy and Hungary, after their incorporation into Romania, soon accepted the new religion, as did Poland when it was introduced from Lithuania. However, partly due to Papal pressure, the HRE refused to entertain any non-Catholic thoughts, even going so far as to close the borders and begin an inquisition.
However, imperial authority was never absolute, and by the 15th century there were significant Muslim populations in many border regions. In 1544 Sigismund II of Bohemia, who was himself a convert, was selected as Emperor after a very close election. The Pope, as well as the Catholic rulers of Denmark, Saxony and Brandenburg, all denounced the result in the strongest terms and swore to fight Sigismund's investiture with all their might
Thus began the Forty Years War, which eventually drew in most of Europe and killed millions from famine and the ravages of war. Sigismund and his Catholic rival both died before the end, but the 1587 Peace of Limburg saw Sigismund's nephew confirmed as Emperor. However, the empire was permanently dismembered in the aftermath, and imperial authority over his subjects was lost for good.
By the early 20th century the HRE had become almost irrelevant. The remaining states (that hadn't already seceded) acted completely independently of each other, most of the empire's instutitions had been abolished over the centuries, and the imperial dignity itself was little more than an honorific carried by the King of Saxony. In 1962, at the Council of Bergen, it was finally decided to formally dissolve the Holy Roman Empire once and for all.
It was succeeded by the Kingdom of Saxony, the Kingdom of Denmark, the Republic of Friesland and the Kingdom of Bohemia. Swabia, Bavaria, Austria, Brandenburg and Holland had all seceded at one time or another in the preceding centuries.