The European balance of power is a tenet in international relations that no single power should be allowed to achieve hegemony over a substantial part of Europe. During much of the Modern Age, the balance was achieved by having a small number of ever-changing alliances contending for power, which culminated in the World Wars of the early 20th century.
The emergence of city-states (poleis) in ancient Greece marks the beginning of classical antiquity. The two most important Greek cities, the Ionian-democratic Athens and the Dorian-aristocratic Sparta, led the successful defense of Greece against the invading Persians from the east, but then clashed against each other for supremacy in the Peloponnesian War. The Kingdom of Macedon took advantage of the following instability and established a single rule over Greece. Desire to form a universal monarchy brought Alexander the Great to annex the entire Persian Empire and begin a Hellenization of the Macedonian possessions. At his death in 323 BC, his reign was divided between his successors and several Hellenistic kingdoms were formed.
Rome expanded into the whole of Italy around the same period and then rose to prominence in the western and Eastern Mediterranean through the Punic Wars and Macedonian Wars, but was then shaken by a century-long political crisis. Meanwhile, the popularity and wealth of Roman generals increased: notably Julius Caesar acquired fame for projecting military power north of the Alps into Gaul, east of the Rhine into Germania and across the Channel into Britain. A group of senators afraid of Caesar's title of dictator for life assassinated him on the Ides of March of 44 BC. The adoptive son of Caesar, Octavian Augustus, defeated the killers of his father and became the first Roman Emperor (Princeps) in 27 BC.
The Roman Empire peaked during the Pax Romana, stagnated during the crisis of the third century AD and ultimately split between the Latin West and the Greek East. Both parts of the Empire abandoned pagan polytheism in order to tolerate monotheistic Christianity and finally make it the state religion. The West collapsed around 476, following centuries of attacks by mainly Germanic peoples and several successor states were established on its former territory. The East continued to be ruled by the Eastern Roman Empire for an additional thousand years.
Among the successor states in the West, that of the Franks was the largest, and under Charlemagne managed to unite most of present-day France, Germany, Switzerland, Austria, the Low Countries and Italy under one rule: he was subsequently crowned Holy Roman Emperor the day of Christmas in 800 by Pope Leo III. This led to a conflict with the Byzantine Empire, the so-called problem of two emperors saw both empires being unable wage war with each other making other disputes and common threats the dominating factor of their relations.
Meanwhile, the Iberian Peninsula fell under Muslim control. The beginning of the Reconquista of Christian forces is traditionally dated to the Battle of Covadonga (718 or 722), in which an Asturian army achieved the first Christian victory over the forces of the Umayyad Caliphate since the beginning of the military invasion. Its culmination came in 1492 with the fall of the Nasrid kingdom of Granada to the united Spanish Crown of Ferdinand II of Aragon and Isabella I of Castile. The Germanic Emperor (Holy Roman Emperor) and the Roman Pontiff (Pope in Rome) came to be known as the universal powers of Europe, but then entered in conflict during the investiture controversy and the clash between their factions. Their rivalry made possible the birth of autonomous city-states in northern Italy and the rise of an independent feudal monarchy in France under the House of Capet. Around the same period, the Norman Conquest of England happened in 1066 and Sicily fell in 1130. With the Holy Land lost to Islam and the Romans seeking help from Turks, the Pope initiated the First Crusade against the Muslims in an attempt to restore Christian authority in former Roman lands following the Eastern Schism of the Orthodox from the Catholics.
Most of the crusades did not achieve their objective, but some of them had a massive impact on the political and economic landscape of Europe: the First Crusade (1099) re-opened the trade routes in the Mediterranean and ushered in the commercial revolution; the Fourth Crusade (1204) resulted in the formation of the Venetian maritime empire; and the Sixth Crusade (1228) temporarily made Frederick II, heir of both the Kingdom of Sicily the Holy Roman Empire, and King of Jerusalem. At the same time, the Reconquista was taking place in the Iberian Peninsula and the kingdoms of Portugal, Castile and Aragon were formed. A vast part of the French nobility took part in the crusades under the leadership of their king: this made possible the formation of a strong centralized French monarchy. The rise of medieval France began with the Battle of Bouvines (1214), the German Interregnum (1250) and the Avignon Papacy (1309) but ended with the outbreak of the Hundred Years' War (1337) with England and the return of the papacy to Rome (1378). After Europe recovered from the Black Death, the goldsmith Johannes Gutenberg invented the movable-type printing press, which started the Printing Revolution. A Renaissance in art and science began in Italy and spread to the rest of the continent.
Portugal formed the first European colonial empire in 1415 with the conquest of Ceuta. In 1453, the French expelled the English from their land, and the Ottoman Turks conquered Constantinople, initiating the dominance of the Ottoman Empire in Europe. At the end of the 15th century, following the marriage of Isabella I of Castile and Ferdinand II of Aragon Spain was dynastically unified and the Reconquista concluded successfully. Portugal and Spain, followed by France and England, ushered in the Age of Discovery. During the early 16th century, France and the House of Habsburg clashed during the Italian Wars. In 1519, Charles V of Habsburg, already Duke of Burgundy, King of Spain, and Archduke of Austria, became Holy Roman Emperor. After the defeat in the Battle of Pavia, Francis I of France allied with the Muslim Ottoman sultan Suleiman the Magnificent. After the annexation of the Aztec Empire and conquest of the Incas, Emperor Charles used the gold and silver coming from the Americas to finance the defence of his German territories in Austria from the Ottoman Empire (Siege of Vienna) and of his Italian territories in the Duchy of Milan from France (Battle of Pavia). In response, European rival states sanctioned privateers to raid Spanish or Portuguese ships full of gold and silver, most especially in the Caribbean. Ultimately, Charles V conceded the Peace of Augsburg and abandoned his multi-national project with a series of abdications in 1556 that divided his hereditary and imperial domains between the Spanish Habsburgs, headed by his son Philip II of Spain, and the Austrian Habsburgs, headed by his brother Ferdinand. Ferdinand had been Archduke of Austria in Charles's name since 1521 and the designated successor as emperor since 1531.
The papacy launched the Catholic revival in an attempt to halt the growth of Protestantism and Ottoman expansion. Despite some successes, such as the Battle of Lepanto (1571) and the Siege of Paris (1590), the Anglo-Spanish War and the Long Turkish War questioned the Catholic ambitions. Ultimately, the papacy lost its status and influence with the Thirty Years' War (1618–1648) where the Catholic French empire allied with the Protestant nations to defeat the Habsburg alliance. The Thirty Years War was one of the longest and most destructive conflicts in European history. Fought primarily in Central Europe, an estimated 4.5 to 8 million soldiers and civilians died as a result of battle, famine, and disease, while some areas of what is now modern Germany experienced population declines of over 50%. Related conflicts include the Eighty Years' War, the War of the Mantuan Succession, the Franco-Spanish War, the Dutch-Portuguese War and the Portuguese Restoration War. Many Protestant states also experienced a golden age: the newly independent Netherlands formed the Dutch East India Company in Indonesia; Sweden formed an empire in northern Europe; and England began the colonization of North America. By the Treaty of Westphalia at the end of the Thirty Years' War, the Holy Roman Empire became a more decentralized entity in which constituent states, such as Prussia (which also had lands outside the Empire), were allowed to pursue their own foreign policy independent of that of the Austrian Habsburg Emperor. The Austrian Habsburgs also controlled some states outside of the Holy Roman Empire. It was France under Louis XIV who took the status of main continental power from the Habsburgs thanks to the Treaty of Westphalia and the Treaty of the Pyrenees.