The most popular explanation for Labour’s decision to abstain and then U-turn on the government’s immigration bill last week – a bill that ends freedom of movement in the European Union, enshrines privileges for high-earning migrants against low-earning ones, and forces all EU citizens to apply for “settled status” – is simple electoral strategy.

If Labour hopes to court Leave voters, the argument goes, it must heed their call to constrain migration. Such was the logic of their 2017 manifesto promise that “freedom of movement will end” after Brexit.

But the Corbyn project is premised on its courage to rise above such crass electoralism. It was his brave decision to oppose the 2015 welfare bill that pushed Corbyn to the head of the race for the Labour leadership. And it was his long history of against-the-grain activism — from the apartheid picket line to the anti-war movement — that earned him the loyalty of the young left.

To explain away the initial abstention on the bill as “just politics” is, then, both too dismissive of Corbyn’s principles and too generous to his political acumen.

Instead, the more accurate explanation situates the Labour Party’s position on migration in the context of Corbyn’s broader worldview, which we might call “Internationalism in One Country,” or IOC: the view that the goals of the working class, both within and beyond Britain, are best served by reclaiming national sovereignty, even if — or precisely because — it comes at the expense of free movement.

Motivating this worldview are the perceived sins of the European Union. This week, several clips of old footage resurfaced to show Jeremy Corbyn railing against the “military Frankenstein” of the EU, guilty of “suppressing the economy and creating unemployment.”

For Labour Eurosceptics, the four “freedoms” enshrined by the Treaty of Rome appear as dangerous doublespeak. Dressed up in the language of liberty, they threaten to incarcerate EU member-states, instead.

Free movement of capital prevents governments from regulating footloose finance. Free movement of goods prevents governments from addressing rapid deindustrialisation. John McDonnell’s early hints at capital controls and Corbyn’s industrial policy speech aimed to curtail these two freedoms, respectively.

But this critique also extends to the free movement of people, or — in the language of the original treaty — “labour.” The EU may offer bromides about European citizenship and community, but the freedom to move is merely another mechanism of capitalist control. “If freedom of movement means the freedom to exploit cheap labour in a race to the bottom, it will never be accepted in any future relationship with Europe,” Corbyn wrote.

Placing Labour’s migration policy in this context helps to resolve some of its apparent contradictions. Corbyn wants both to uphold migrants’ rights in Britain and challenge the system that sent them there. Corbyn wants both to expand Britain’s assistance to asylum-seekers and contract its scope for “economic migration.”

If the old internationalists believed that integration was the key to progress — “breaking down national barriers and prejudices, uniting workers from all countries,” as Lenin once wrote — IOC advocates believe the opposite. Free movement is “the opposite of what is left-wing,” says Die Linke MP and fellow IOC advocate Sahra Wagenknecht. “All successes in restraining and regulating capitalism have been achieved within individual states, and states have borders.”

But the basic claims that undergird their critique of EU free movement don’t stand up to scrutiny.

Consider the economic argument that migrants undercut wages and take native jobs. Most empirical evidence suggests “little or no impact on average employment or unemployment” in the UK, with migrants contributing more in taxes than they take in benefits.

It is true that any impact is concentrated among low-skill, low-education workers — the most vulnerable in the labour market. But it is entirely unclear why the solution would be to restrict migration, rather than enforcing higher labour standards for all.

If anything, hard borders only serve to create an underclass of “unauthorised” migrants that are far more vulnerable to exploitation than EU citizens protected by European law.

The cultural argument against free movement is similarly suspect.

Some critics, like Paul Mason, argue that “it says to people with strong cultural traditions, a strong sense of place and community…that ‘your past does not matter’.”

But time and again, it is areas with the fewest migrants that shout loudest on behalf of cultural preservation. Restricting migration in order to appease their nostalgia would therefore be futile at best, and counterproductive at worst. After all, have President Trump’s ICE deportation raids reduced white Americans’ sense of cultural resentment, or further incensed it?

The political argument against free movement strikes at the core of IOC. Nation-states require sovereignty: the ability to make decisions according to the democratic will of their voters. The Labour leadership therefore promises to accommodate asylum-seekers fleeing humanitarian crisis, but reserves the right to regulate the inflow of economic migrants in order to serve — per the manifesto — “our economic needs.”

But it is here that IOC advocates sound most like the neoliberals they claim to loathe. In making this case for state sovereignty, they make migrants sound like nothing more than lumps of labour.

In truth, the distinction between asylum-seekers and “economic migrants" is false, and flagrantly so. In Athens, where I live, young people arrive in droves to seek a better life in Europe. But the EU only grants asylum to those it deems sufficiently desperate; economic migrants, often fleeing violence and persecution in countries in sub-Saharan Africa and the Middle East, are turned away.

There are certainly reasons to criticise the institution of EU free movement. For one, it is only a partial freedom, restricted by class, ethnicity, and geography. For another, it is an extremely exclusive one, leaving thousands of migrants to perish on the Mediterranean each year.

But it is dangerously foolish to believe that the Labour Party can destroy EU free movement in order to build something better in its place. As we are about to discover, the costs of ending free movement — to the millions of EU citizens that have made homes, families, and lives in Britain — are painfully high. Allowing this to pass will be a permanent stain on the Labour Party’s record.

Instead, EU free movement should be a springboard to a more global system of open migration. Labour should be leading the way to defend EU free movement and extend it beyond Europe’s borders, shattering the fortress that Brussels has constructed around the continent.

This is just the beginning. Climate change will force many more people across the world to migrate, and by 2100, an estimated one million migrants will travel to Europe each year. If Labour fall into the trap of IOC and enable Britain’s borders to harden, they will condemn these migrants to misery, and in doing so impoverish the nation’s rich and diverse community in the process.

David Adler works for Yanis Varoufakis as the policy coordinator for Diem25. He tweets @davidrkadler