When I visited Europe in 2008 and before, the idea that Europe was not going to emerge as one united political entity was regarded as heresy by many leaders. The European enterprise was seen as a work in progress moving inevitably toward unification - a group of nations committed to a common fate. What was a core vision in 2008 is now gone. What was inconceivable - the primacy of the traditional nation-state - is now commonly discussed, and steps to devolve Europe in part or in whole (such as ejecting Greece from the eurozone) are being contemplated. This is not a trivial event.
Before 1492, Europe was a backwater of small nationalities struggling over a relatively small piece of cold, rainy land. But one technological change made Europe the center of the international system: deep-water navigation.
The ability to engage in long-range shipping safely allowed businesses on the Continent's various navigable rivers to interact easily with each other, magnifying the rivers' capital-generation capacity. Deep-water navigation also allowed many of the European nations to conquer vast extra-European empires. And the close proximity of those nations combined with ever more wealth allowed for technological innovation and advancement at a pace theretofore unheard of anywhere on the planet. As a whole, Europe became very rich, became engaged in very far-flung empire-building that redefined the human condition and became very good at making war. In short order, Europe went from being a cultural and economic backwater to being the engine of the world.
At home, Europe's growing economic development was exceeded only by the growing ferocity of its conflicts. Abroad, Europe had achieved the ability to apply military force to achieve economic aims - and vice versa. The brutal exploitation of wealth from some places (South America in particular) and the thorough subjugation and imposed trading systems in others (East and South Asia in particular) created the foundation of the modern order. Such alternations of traditional systems increased the wealth of Europe dramatically.
But "engine" does not mean "united," and Europe's wealth was not spread evenly. Whichever country was benefitting had a decided advantage in that it had greater resources to devote to military power and could incentivize other countries to ally with it. The result ought to have been that the leading global empire would unite Europe under its flag. It never happened, although it was attempted repeatedly. Europe remained divided and at war with itself at the same time it was dominating and reshaping the world.
The reasons for this paradox are complex. For me, the key has always been the English Channel. Domination of Europe requires a massive land force. Domination of the world requires a navy heavily oriented toward maritime trade. No European power was optimized to cross the channel, defeat England and force it into Europe. The Spanish Armada, the French navy at Trafalgar and the Luftwaffe over Britain all failed to create the conditions for invasion and subjugation. Whatever happened in continental Europe, the English remained an independent force with a powerful navy of its own, able to manipulate the balance of power in Europe to keep European powers focused on each other and not on England (most of the time). And after the defeat of Napoleon, the Royal Navy created the most powerful empire Europe had seen, but it could not, by itself, dominate the Continent. (Other European geographic features obviously make unification of Europe difficult, but all of them have, at one point or another, been overcome. Except for the channel.)
The tensions underlying Europe were bought to a head by German unification in 1871 and the need to accommodate Germany in the European system, of which Germany was both an integral and indigestible part. The result was two catastrophic general wars in Europe that began in 1914 and ended in 1945 with the occupation of Europe by the United States and the Soviet Union and the collapse of the European imperial system. Its economy shattered and its public plunged into a crisis of morale and a lack of confidence in the elites, Europe had neither the interest in nor appetite for empire.
Europe was exhausted not only by war but also by the internal psychosis of two of its major components. Hitler's Germany and Stalin's Soviet Union might well have externally behaved according to predictable laws of geopolitics. Internally, these two countries went mad, slaughtering both their own citizens and citizens of countries they occupied for reasons that were barely comprehensible, let alone rationally explicable. From my point of view, the pressure and slaughter inflicted by two world wars on both countries created a collective mental breakdown.
I realize this is a woefully inadequate answer. But consider Europe after World War II. First, it had gone through about 450 years of global adventure and increasingly murderous wars, in the end squandering everything it had won. Internally, Europe watched a country like Germany - in some ways the highest expression of European civilization - plunge to levels of unprecedented barbarism. Finally, Europe saw the United States move from the edges of history to assume the role of an occupying force. The United States became the envy of the Europeans: stable, wealthy, unified and able to impose its economic, political and military will on major powers on a different continent. (The Russians were part of Europe and could be explained within the European paradigm. So while the Europeans may have disdained the Russians, the Russians were still viewed as poor cousins, part of the family playing by more or less European rules.) New and unprecedented, the United States towered over Europe, which went from dominance to psychosis to military, political and cultural subjugation in a twinkling of history's eye.
Paradoxically, it was the United States that gave the first shape to Europe's future, beginning with Western Europe. World War II's outcome brought the United States and Soviet Union to the center of Germany, dividing it. A new war was possible, and the reality and risks of the Cold War were obvious. The United States needed a united Western Europe to contain the Soviets. It created NATO to integrate Europe and the United States politically and militarily. This created the principle of transnational organizations integrating Europe. The United States also encouraged economic cooperation both within Europe and between North America and Europe - in stark contrast to the mercantilist imperiums of recent history - giving rise to the European Union's precursors. Over the decades of the Cold War, the Europeans committed themselves to a transnational project to create a united Europe of some sort in a way not fully defined.