That the 10-year anniversary of Brexit coincided with the fall of another British government is a fitting marker of a failed experiment. Now, the question for the UK and the European Union is less about what to do next than whether they can summon the will to do it. As the owner of a business that is growing in both places, and as an American who recognizes that the U.S. benefits from their success, I’m hopeful they can.

It was clear in 2016 that exiting the EU would be a grave error. A decade later, the costs are clearer than ever — and they keep piling up. The project has delivered none of the promised economic benefits and nearly all of the foreseeable downsides, plus a level of political volatility — six, soon to be seven, prime ministers in 10 years — unprecedented in modern British history.

Today, a large majority of young voters favor rejoining the EU, and polls show most Britons recognize leaving was a mistake. No wonder, given the results.

If the goal of Brexit was a leaner, more competitive Britain, the data is clear that it failed. The UK’s Office for Budget Responsibility assumes Brexit will leave productivity 4% lower over the long run than it would’ve been under EU membership. Other studies indicate that it has already reduced the UK’s gross domestic product. Not surprisingly, businesses most exposed to the EU through trade, labor or ownership have fared the worst.

The promised regulatory benefits were also a mirage. Britain still mirrors much EU regulation, only without a vote over its design. Estimated gains from new trade deals are small and swamped by the costs.

Nor did Brexit end high levels of migration, as many Brexiteers promised. In fact, just the opposite happened. While net migration from the EU fell sharply, non-EU net migration surged from about 200,000 before Brexit to nearly 1 million at its peak in 2023, and it remains above its pre-Brexit levels. Undocumented arrivals also remain far higher than they were pre-Brexit, and the UK has lost direct access to some of the EU’s border security and policing tools, which has hampered efforts to stop small boat crossings.

On top of all that, Brexit has consumed years of focus from government officials, wasting time and energy that could’ve been directed toward the country’s real needs: greater investment, higher productivity and reform of public services. The many challenges the new prime minister will now face are all harder than they otherwise would be, in no small part because the country is poorer than it otherwise would be.

Europe also lost. A bloc now obsessed with competitiveness, security and strategic autonomy is weaker without the full weight of Britain’s financial prowess, military resources and diplomatic strength. And the world has moved in ways that make the split more costly by the year. Russia is more dangerous and China more powerful, while the UK’s special relationship with the U.S. is less certain. In that environment, intra-European friction is a luxury neither side can afford.

The possibility of Britain rejoining a reformed, more competitive EU is no panacea, but it is a goal that both sides should work toward, given the major benefits each would reap. Still, it is many years off. In the meantime, tangible integration gains are still achievable if both sides exercise the will.

That should start with greater investment in security by both the UK and the EU. Europe’s military spending has increased but is still below NATO targets, and its effectiveness has been undermined by fragmentation and poor procurement. Closer cooperation between the EU and the UK on joint procurement, replenishing weapons and munitions stockpiles, cyber resilience, and defense industrial capacity would benefit both sides.

The UK and the EU can also reduce economic friction. Ease border checks to help exporters. Link carbon emissions trading systems to reduce compliance costs. Coordinate electricity markets to lower bills, improve resilience and support decarbonization. Such gains have long been promised, but there’s too little to show for it.

Reducing economic friction should also involve greater labor market cooperation, starting with young people. Young Britons have lost the most from Brexit and know they are suffering from it. More than 80% say they’d rejoin the EU, and they should not be blocked from exploring educational and career opportunities there.

At the same time, greater access to the largest English-speaking market on the EU’s doorstep would offer European young people valuable work and language experience. Debates over the details — how many should be eligible, how long should they be allowed to stay, how much tuition should be charged — should not be insurmountable if both sides show good faith.

Such reforms will not come close to undoing the damage caused over the past 10 years, but they would constitute the kind of progress that may allow a government to survive more than a year or two — and for both the UK and the EU to begin putting themselves on firmer footing.

I remain bullish on their prospects for success, given their underlying strengths. But the past decade has shown, and recent geopolitical conflicts have underscored, how dangerous it is for any country or region to attempt to go it alone. Democratic nations grow in strength — economically, politically, militarily and culturally — when we grow together. And as the democratic values and security concerns that bind the UK and the EU together come under assault, a close partnership is more essential than ever.

Now that the fog of the Brexit debate has lifted and the damage is plain to see, the question is whether Britain and Europe are prepared to repair their relationship to rebuild their futures. The fate of the new government now forming in London hinges on the answer.

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Michael R. Bloomberg is the founder and majority owner of Bloomberg LP, the parent company of Bloomberg News, and the founder of Bloomberg Philanthropies.

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