The Roman empire historically was not Europe as such. It overtly included Turkey (in Asia) and North Africa. So I see no warrant to focus on Europe as such. I don't know that Rome exercised that much authority in Germany (which was not a nation then)
The Western Roman Empire, which began its terminal collapse in 408, was notionally reborn nearly four centuries later through translatio imperii, the transfer of its lands, to Charlemagne, Charles the Great, King of the Franks, who had been crowned Emperor of the Romans on Christmas Day 800 by Pope Leo III.[26][27]
In 955, Otto won a decisive victory over the Magyars in the Battle of Lechfeld.[30]:707 In 951, Otto came to the aid of Adelaide, the widowed queen of Italy, defeating her enemies, marrying her, and taking control over Italy.[31]:214–15 In 962, Otto was crowned Emperor by Pope John XII.[30]:707 From then on, the affairs of the German kingdom were intertwined with those of Italy and the Papacy. Otto's coronation as Emperor made the German kings successors to the Empire of Charlemagne, which through translatio imperii, also made them successors to Ancient Rome. Additionally, in 963, Otto deposed the current pope John XII and chose Pope Leo VIII as the new pope (although John XII and Leo VIII both claimed the papacy until 964, when John XII died).
It was his second son, Henry V, who managed to reach an agreement with both the Pope and the bishops in the 1122 Concordat of Worms.[35]:123–34 The political power of the Empire was maintained, but the conflict had demonstrated the limits of any ruler's power, especially in regard to the Church, and it robbed the king of the sacral status he had previously enjoyed. Both the Pope and the German princes had surfaced as major players in the political system of the Empire.
Charles V[a] (24 February 1500 – 21 September 1558) was ruler of the Holy Roman Empire from 1519 and, as Charles I, of the Spanish Empire from 1516 until his voluntary abdication in favor of his younger brother Ferdinand I as Holy Roman Emperor and his son Philip II as King of Spain in 1556.
Francis II (German: Franz II, Erwählter Römischer Kaiser) (12 February 1768 – 2 March 1835) was the last Holy Roman Emperor, ruling from 1792 until 6 August 1806, when he dissolved the Holy Roman Empire after the disastrous defeat of the Third Coalition by Napoleon at the Battle of Austerlitz. In 1804, he had founded the Austrian Empire and became Francis I (Franz I.), the first Emperor of Austria (Kaiser von Österreich), ruling from 1804 to 1835, so later he was named the one and only Doppelkaiser (double emperor) in history
Germany, or more exactly the old Holy Roman Empire, in the 18th century entered a period of decline that would finally lead to the dissolution of the Empire during the Napoleonic Wars. Since the Peace of Westphalia in 1648, the Empire had been fragmented into numerous independent states (Kleinstaaterei). In 1701 Elector Frederick of Brandenburg was crowned "King in Prussia". From 1713 to 1740, King Frederick William I, also known as the "Soldier King", established a highly centralized state.
Here is an interesting article that illustrates how a state and church could work together...
Adolf Hitler, Charlemagne, and the Holy Roman Reich