The last question is a real fear for some. Last week, Germany's foreign minister warned that a Brexit would put Europe's "decades-long, successful integration effort" at risk of "disintegration."

The European Union is unique in history with its embrace of liberal, democratic values. After centuries of wars and bloodshed, here was the solution for a lasting, fruitful peace: a confederation of European states, most of which would also agree to open borders and a joint currency. That picture looks less rosy now, given the growing traction of Euroskeptic politics in many parts of the continent, not least in Britain.

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This is hardly the first time, of course, that a great continental political project has faltered or collapsed. WorldViews charts almost two millennia of European history through some of its most ambitious empires and alliances.

Roman Empire

At its height, the Roman Empire stretched from the windswept moors of northern England to the desert wastes of Syria. Under the Pax Romana, much of what’s now Europe was knitted together by roads, bureaucracy and politics. A common language – Latin – was chiseled on monuments and temples across the realm. Cults to eastern gods – Mithras, for example, or Isis – spread far and wide. Men born in places such as Spain and the Balkans could aspire for glory and prominence in Rome.

The empire was a continental power where before none had existed, and even after its collapse and disarray by the 5th century A.D., its imprint had a profound hold on the course of European history.

The kingdom of Charlemagne, 814

Under the Frankish warlord Charlemagne and his Merovingian predecessors, Western Europe saw its first marked political integration since the waning of Rome. The Franks subdued Germanic tribes in forests east of the Rhine and battled Muslim armies pushing north from Spain. From his capital at Aachen, in what’s now Germany, Charlemagne ruled over a vast domain that at its peak may have encompassed 10 million to 20 million people. An intervention into papal politics saw him crowned as “emperor of the Romans” in the Basilica of St. Peter’s on Christmas Day, A.D. 800 – a resurrection of the Roman Empire in the west in title, if not practice, that lingered for 1,000 years. After Charlemagne’s death in 814, his empire would dissolve amid competing factions, but the political geography of much of Western Europe, including the distinct kingdoms of France and those in Germany, was set.

Byzantine Empire

Although the power of Rome dimmed in the west, the empire's legitimacy was preserved in the east with the Byzantines. They were at once the paramount power in the Mediterranean, as well as the main bulwark between lands steadily claimed by Islam and Christendom in the west. The Byzantine capital of Constantinople, now Istanbul, was a thriving center of art and trade and perhaps the most important bastion of Christian rule until its fall to besieging Ottomans in 1453. Yet the Greek-speaking and Greek Orthodox Byzantines were often at odds with the realms to the west. In 1204, more than two centuries before the Ottomans took Constantinople, a rampaging Crusader army sacked the city, carrying out a brutal campaign of rape, plunder and destruction.

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"All places everywhere were filled full of all kinds of crime," wrote one chronicler at the time. "Oh, immortal God, how great the afflictions of the men, how great the distress!"

The realms of the Normans

The Normans were the descendants of Viking raiders who were bequeathed land to settle in northern France -- a region now known as Normandy -- by the king in Paris in the 10th century A.D. Not long thereafter, Norman lords were opportunistically establishing kingdoms in various corners of Europe. William the Conqueror famously wrested control of England in 1066. Norman knights laid siege to the ancient city of Antioch during the First Crusade, and remained in their Holy Land strongholds for decades thereafter. And in 1130, with papal recognition, a Norman lord was hailed King of Sicily--in control of a poly-glot, multi-faith Mediterranean realm that survived for almost two centuries.

The Hanseatic League

The Hanseatic League was Europe's first embryonic attempt at a jointly policed free trade zone. Guilds and powerful merchant families in northern Europe banded together to form associations that linked a host of cities together, centered around influential German ports on the Baltic Sea. In some circumstances, they shared common laws and operated in mutual defense. The league's heyday was between the 13th and 15th centuries, but its monopoly over trade and ship-building in the region collapsed amid turbulent shifts in Europe's late medieval landscape, including the Protestant Reformation and growth of rival power centers in Russia, Sweden and what's now the Netherlands.

Hapsburg Europe under Charles V

Charles V was emperor of quite a lot. The heir to three powerful European dynasties, he ended up through luck of succession in command of Hapsburg holdings in central Europe, parts of the Netherlands, and both Spain's European domains as well as its vast colonial territories in the Americas. He was the Archduke of Austria, the Holy Roman Emperor, and the King of Spain. Born in Ghent, now Belgium, he was also a quintessential European. A famous witticism attributed to him: "I speak Spanish to God, Italian to women, French to men and German to my horse." Wars on numerous fronts, including battles with the Ottomans and France, hobbled his reign and he ended up abdicating the throne in 1556 and retiring to a monastery. His son, Philip, took charge of the Spanish empire, while his younger brother Ferdinand assumed the Hapsburg mantle in central Europe.

The Polish-Lithuanian Commonwealth

The Polish-Lithuanian Commonwealth is the largest European state that you probably never heard of. At its peak, the bi-republic -- a merging of the Kingdom of Poland and the Grand Duchy of Lithuania -- encompassed a vast tract of what is now Eastern and Central Europe, spanning the Baltic States, Poland, Belarus and Ukraine and held together, to varying extents, between the end of the 14th century until the end of the 18th century. It was marked by an unusual religious tolerance; for a time, 80 percent of the world's Jewry lived within its borders. Some historians consider its constitution, issued in 1791, to be the first such modern, proto-democratic document drafted in Europe. The commonwealth was carved up and eventually subsumed by the expansion of three rival empires at its borders: Russia, Prussia and Hapsburgs of Austria-Hungary.

Ottoman Europe

In 1683, a mighty Ottoman army was repulsed at the gates of Vienna for a second time. The failed siege of the Austrian capital is considered a defining moment in European history -- the pivotal battle that shielded the realms of Christendom from the advance of the Muslim Ottomans. For centuries, though, the Ottomans were as "European" as anyone else. Its armies and senior leadership were populated by janissary recruits and Christian converts from across its borderlands along the Mediterranean and the Black Sea. The attack on Vienna, moreover, had been planned in league with the king of France -- the Ottomans were European power-brokers like any other major empire. In the centuries that followed, Istanbul's European domains would steadily slip away until the empire's dissolution after World War I.

Europe under Napoleon

In June 1812, Napoleon Bonaparte, the crowned French emperor, led his Grand Army on what would be doomed invasion of Russia. The catastrophe of the expedition -- which saw hundreds of thousands of deaths -- would lead to the unraveling of his conquests through much of Europe and his brief exile to the Mediterranean isle of Elba. Before that, though, within less than a decade, the Corsican general had won an astonishing series of battles that brought much of Europe either under his direct control or in alliance with his interests. Europe would not see such a rapid, expansionist project until World War II.

Europe on the eve of World War I

World War I was a conflict that followed decades of secret alliances and scheming between the continent's great powers. The Triple Entente -- a defense pact between Britain, France and later Russia -- vied against the interests of Germany and the Austria-Hungarian Empire. Eventually, the Ottoman Empire would enter the war against its longtime foe in Russia and Italy, Romania and Serbia would also side with the Triple Entente powers in a bid to take on the Austrians.

Nazi Europe

At its height, the Third Reich and its Axis allies controlled a huge chunk of Europe from France to the blood-soaked borderlands of the Soviet Union. Nazi expansionism was fueled by Adolph Hitler's desire for "lebensraum," or living space for the Germanic race. It turned stretches of the continent into a hideous charnel house where millions perished.

The Benelux Union

A political and economic union between three relatively small, neighboring states -- Belgium, the Netherlands and Luxembourg -- the Benelux group would join West Germany, France and Italy to form the European Coal and Steel Community, a precursor to the organizations that would ultimately lead to the creation of the European Union. The shared franc currency between Belgium and Luxembourg can also be seen as a precursor to the currency shared by the euro zone.

NATO before the fall of the U.S.S.R.

Before the demise of the Soviet Union, the North Atlantic Treaty Organization was the paramount military alliance of the West. It remains so, even after the end of the Soviet threat. Now billed as an alliance of democracies, it initially stitched together nations that were anything but -- including 20th century anti-communist dictatorships in Greece and Turkey. The continued prospect of NATO expansion, which encompasses many more nations than those seen on the map above, riles Moscow to this day.

The European Union, now

Meanwhile, the European Union, officially declared in 1993, also grew, and now counts 28 states as its members. But after Britain votes on Friday, will that number shrink?

The story has been updated with new maps.