This is what's riding on the EU referendum

In 1963, French President Charles de Gaulle stunned the United Kingdom by rejecting its application to join the European Economic Community, the predecessor of the European Union. The logic behind de Gaulle’s famous “non” was simple: Britain was not sufficiently European.

“England in effect is insular, she is maritime, she is linked through her exchanges, her markets, her supply lines to the most diverse and often the most distant countries,” explained de Gaulle. “It is possible that one day England might manage to transform herself sufficiently to become part of the European community…. In this case…France would raise no obstacle.”

De Gaulle’s veto held for as long as he lived; it was not until 1973, under his successor Georges Pompidou, that France lifted its objections to British membership. In the more than 40 years since, the UK has played a major role in shaping the course of European integration, while transforming itself from a “sick man of Europe” into one of the world’s most competitive economies.

Few today remember that it was UK Prime Minister Margaret Thatcher, despite her vocal euroskepticism, who restarted Europe’s integration process after a decade in which it had stagnated. Thatcher’s ally, Arthur Cockfield, EU Commissioner for the Internal Market and Services, led the push for a truly integrated market for goods, services, people, and capital – an effort that ultimately led to the creation of the EU Single Market in 1992. Thatcher even broke the old Gaullist dictum that each EU member must hold a national veto on all decisions, paving the way for majority voting.

Likewise, few European politicians have argued more eloquently for a truly common European foreign and security policy than former UK Prime Minister Tony Blair. For him, preserving peace was certainly important, but making Europe a global player was the priority.

So it is ironic that a significant swath of the British electorate seems to share de Gaulle’s verdict on their country’s affinity to Europe. On June 23, in a decision of momentous importance for all of Europe, UK voters will decide in a referendum whether to exit the EU. If they choose to leave, they risk not only unraveling their own economic successes, but also destroying the very underpinnings of a unified Europe.

A British exit – or Brexit – would cause severe damage to the entire continent. In the 1970s and 1980s, the magnetic promise of integration helped stabilize democracy in Greece, Spain, and Portugal. In the 1990s, when ten countries and 100 million citizens broke from the Soviet empire and joined the West, the promise of EU accession eased, encouraged, and to some extent guided the transition. The soft power of an integrated Europe inspired democratic reform for decades in Turkey; and only two years ago, the promise of Europe inspired democratic change in Ukraine. Although both cases reveal the limits to the EU’s soft power, it remains the key to overcoming the legacies of strife in the Balkans.

If the UK leaves, that power will quickly wane. Other, grimmer models will become more powerful. The demons of history have yet to be definitively banished in Europe. And a Europe that begins to fracture would not only be weaker; its vulnerability to the destabilizing forces already gathering within its borders would make it more dangerous as well.

Only by acting together can European countries secure the continent’s stability and, to some extent, that of its adjacent neighborhood. Without the UK as a central part of its structure of peace, Europe may simply lack the necessary mass and begin to spin apart.

At a minimum, Brexit would throw the EU into years of uncertainty. Negotiating a complicated divorce and a new relationship with Britain could consume the EU’s political oxygen (especially if, as is quite possible, the UK itself breaks up, with Scotland rejoining Europe). This would distract Europe from other serious challenges such as Russian aggression, instability in the Middle East, and its own moribund economy.

For the United States, Brexit would be a betrayal of a key element of foreign policy championed by every American president since Dwight D. Eisenhower. Meanwhile, Russian President Vladimir Putin would certainly rejoice. And nationalist forces across Europe would suddenly feel that the future is theirs.

Advocates of Brexit are remarkably reluctant to explain with any precision their plans for their country’s future. Their vision seems almost entirely defined by what it opposes. A key question is whether a post-European UK could remain part of Thatcher’s creation, the single market, especially as it is extended to the digital domain and augmented with free-trade treaties around the world. Leaving the single market would cause grave uncertainty for the British economy, especially its financial sector. Even the US has said that it would be unwilling to negotiate a separate free-trade deal with the UK.

Remaining in the single market, however, comes at a cost – one that British voters may not support. The UK would have to adopt the sort of satellite status that Norway, Iceland, and Liechtenstein now have, accepting and implementing decisions that it takes no part in shaping. If that democratic deficit is unacceptable to the UK, it will be left alone, out in the economic cold.

To be sure, de Gaulle was not mistaken when he pointed out that Britain’s political culture was different from France’s. But the same is true of Sweden’s, Poland’s, or Austria’s. The European project is not about denying diversity or trying to force everyone into the same mold. Indeed, Europe’s diversity is in many ways its greatest strength.

Much rides on British voters’ decision in June. A UK that cuts itself adrift would be at risk of tragedy. A fractured EU would unleash untold dangers. And a world without a strong, unified Europe would be poorer and less safe.