A boat flying a large "In" flag, campaigning to remain in the E.U. in the upcoming referendum sails by the British Houses of Parliament on Wednesday. (Niklas Halle'n/AFP/Getty Images)

Is history on the European Union’s side? It depends on what you mean by “history.” The history of the 20th century, in which Europe nearly destroyed itself in a series of bloody wars among its rivalrous states, provided the rationale for the European project: Peace and prosperity require nations to cede ever-increasing measures of sovereignty to a supranational body.

Many Europeans, enjoying the benefits of borderless travel and commerce, still find that idea compelling today.

However, if you extend the historical frame of reference deeper into Europe’s past — and shift the geographical context eastward toward the Ural Mountains — you find repeated attempts at multinational confederations that sooner or later broke up.

The Holy Roman and Austro-Hungarian empires, the Soviet Union, Yugoslavia and even Czechoslovakia — all came to grief at least in part due to irrepressible conflicts between their component peoples and out-of-touch central authorities.

The latter view will surely gain credence if Britain votes to leave the European Union on June 23, as seems increasingly likely but not yet certain. Even if “Remain” wins, the fact that “Brexit” gained traction at all constitutes an indictment of the E.U., one that political leaders everywhere ignore at their peril.

On June 23, Britain faces a fateful decision: whether or not to leave the European Union. And the world will be watching. (Daron Taylor,Jason Aldag,Danielle Kunitz/The Washington Post)

World War II and the Cold War are distant memories now. What worries Britons is not age-old continental rivalries but the multiple here-and-now crises of security, economic growth and mass migration over which the E.U. has presided, and with respect to which, many plausibly believe, their voices and votes matter not.

And those concerns — about both the E.U.’s performance and its democratic legitimacy — are widely shared across Europe.

Though still viewed positively by a within-the-margin-of-error majority — 51 percent to 47 percent — the E.U.’s favorability rating has been trending lower among its citizens for more than a decade, and it plunged during the past year, according to a new Pew Research Center survey of 10 European countries encompassing 80 percent of the union’s population.

Overwhelming majorities disapprove of the E.U.’s handling of the refu­gee issue; only the citizens of Germany and Poland approve of its economic record.

Given a choice between returning power to their national governments and transferring more to the E.U., Europeans were more than twice as likely to choose the former.

Hostility to the E.U. is strongest in two countries: Greece, which is not surprising, given that country’s predicament, and France, whose population now registers 61 percent disapproval, despite Paris’s historic role in creating it.

These numbers suggest that a vote for Brexit could prove contagious, with disruptive repercussions wider than analysts currently assume.

None of this is to imply the E.U., held together by law, moral suasion and enlightened self-interest, is morally or politically equivalent to monarchies or communist dictatorships of the past.

Yet as a matter of realpolitik, the E.U.’s reliance on “soft power” for cohesion has mutated into a source of instability: Britain’s secessionist referendum is perfectly lawful under the foundational Lisbon Treaty’s Article 50, which is not the sort of escape clause a truly eternal solution to Europe’s ancient rivalries would have included.

If Britain does leave, it will negotiate with the E.U. over separation terms. The latter will face a dilemma: Dealing punitively with Britain might deter copycat exiters but also contradict the non-coercive principles upon which E.U. legitimacy depends.

Getting 27 nations to forge a common approach to Britain, and the accompanying debate, diplomacy and drama, could distract the E.U.’s already overburdened politicians, eroding their ability to govern effectively on migration, economics and security — which would play into the hands of anti-E.U. populists.

Meanwhile, Brexit might administer a grave, and to other Euroskeptics, exemplary, self-inflicted wound for Britain, as many warn (or secretly hope). Or it might not. Would it weaken the pound? Maybe, but core inflation in Britain is only about 1 percent at present, implying manageable, not catastrophic, risk.

Would growth come to a standstill? A comprehensive study by Open Europe suggests that Britain’s economy would be roughly the same size by 2030 with Brexit as it would have been without it, assuming, plausibly, some sort of trade agreement with the E.U.

The ray of hope in all of this, for the E.U. and its leaders, is that support for the union is strongest among the young; a majority of 18-to-34-year-olds view the E.U. favorably in every country but Greece, according to Pew.

If Europe’s rising generation still has faith, this may give the E.U. time and space to adapt and survive, with or without Brexit.

There’s just one catch: Due to years of low birthrates, the youth cohort is dwindling as a share of Europe’s population. The median E.U. citizen is more than 42 years old.

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