First of all, I assume that you mean "Europe Outside of the Roman Empire" because the largest percentage of Roman territory was in Europe, Rome (the city and capital) is in Europe, and the over 20 current European countries were part of the Roman Empire.
Europe outside of the Empire was uncivilized territory, that is to say that the people lived in Tribes that moved around the wilderness and settled in camps. They certainly interacted with each other and with the Romans and could confederate and produce armies, but there was no unifying government because there was no need (all politics could be managed locally) and it would be difficult to maintain (without easy long-distance communication, empires disintegrate).
Europe is not a single country. It's made up of many independent countries, each with their own leader.
Europe is a collection of separate countries. There was no Prime Minister of Europe. The nearest that Europe got to having a single leader during World War 2 was Adolf Hitler who was ultimately the leader of all of occupied Europe.
Europe does not have a leader. Europe is not a country. Europe is a continent of over 50 independent countries, each with their own leaders. So there is no single leader of Europe.
Diocletian.
The Euro.
There was no single leader during WW1. Each country had their own leader.
Western Europe was not a single kingdom during 800 AD. Most of Western Europe was united under King Charlemagne of the Carolingian Empire in 800 AD. However, upon his death, the empire crumbled.
a large territory in which several groups of people are ruled by a single leader or government
Zarathustra.
The major Christian empire that controlled much of Europe during the Middle Ages was the Holy Roman Empire. Established in the early medieval period, it aimed to unify Christian territories under a single ruler, often an emperor crowned by the Pope. The empire played a significant role in the political and religious landscape of Europe, influencing governance, culture, and the spread of Christianity until its dissolution in the early 19th century.
He united many of the tribes of central Western Europe into a single empire
As far as I am aware Islam does not have a single spiritual leader, in the same way that Christianity has the Pope. Until the end of the Ottoman Empire (1918) the sultan of the Ottoman Empire was the de facto spiritual leader of Islam.