LONDON — Britain’s Brexit revolutionaries have victory within their reach — but they may yet throw it all away because of the impossible choice buried in Theresa May’s divorce deal.

The prime minster has negotiated a draft treaty that ends Britain’s membership of the EU but leaves unresolved the Brexit the U.K. will get after March next year: Britain eventually free but set apart from Northern Ireland, or a United Kingdom shackled to the EU.

In other words, May still hasn’t clearly defined what Brexit really means.

Forget the painful compromises and payoffs littered throughout the 585-page draft Withdrawal Agreement, the fact that May could not resolve her Irish dilemma but kicked a softened version into the next phase of negotiations is both her most significant achievement — and her principle negotiating failure.

It offers a chance to make Brexit a reality without the damage leaving without a deal would entail, but it does not remove its most crushing internal contradiction. Because of that, it could even push the true believers in her party to reject this opportunity to leave the EU.

Should Britain choose to end free movement, it will have to leave the single market, with Northern Ireland remaining behind.

In conversations with officials, diplomats and ministers in London, Dublin and Belfast over the past week, May’s deal has been described as the “Irish Free State” option. Her draft agreement gives Britain “the freedom to achieve freedom,” as Irish revolutionary leader Michael Collins said of the 1921 Anglo-Irish Agreement which established the Irish Free State as a U.K. dominion within the British Empire.

The price Britain would have to pay for freedom is the same as that the U.K. offered to Collins: Northern Ireland.

The backstop clause baked into the Withdrawal Agreement means Northern Ireland will remain in a deeper customs union with Brussels than the rest of the U.K., subject to single market regulation over which it has no say.

All that remains in place “unless and until” a future relationship is agreed that maintains the open border with Ireland that the backstop is designed to preserve.

That clause sets up the fundamental Brexit choice: The U.K. can either maintain the full union of the four British nations under Westminster’s authority — or it can end freedom of movement, a key motivator for Leave voters worried about immigration. It can’t do both.

Freedom to achieve freedom

Should Britain choose to end free movement, it will have to leave the single market, with Northern Ireland remaining behind, in a different regulatory zone to the rest of the United Kingdom.

If instead, it chooses to maintain the full integrity of the U.K., freedom of movement must continue — because doing so would require England, Scotland, Wales and Northern Ireland to stay together in a rule-taking relationship with Brussels inside the single market and a tightly bound customs union.

As long as Brussels continues to reject May’s “third way” Chequers compromise of partial single market membership for the whole of the U.K., the Irish dilemma cannot be avoided.

“Northern Ireland is the hostage in this,” as one Democratic Unionist Party senior official said. “Will we be sacrificed for [Great Britain] to get out or not?”

For many MPs the choice is unacceptable.

But given the political reality in Brussels and the parliamentary arithmetic in Westminster, to reject the choice altogether risks jeopardizing the very thing Brexiteers have fought their entire careers to achieve: an exit from the EU.

According to government ministers, Labour frontbenchers and anti-Brexit Conservative MPs, parliament will simply not allow the government to take Britain out of the EU without a divorce agreement, should the one agreed to by May be rejected. “There isn’t a majority in the House of Commons to allow [no-deal] to take place,” said newly reappointed minister Amber Rudd Wednesday.

Despite Brexiteers’ insistence that no deal is the “default,” Labour and Conservative MPs — including a number of ministers — are confident that any responsible U.K. government would put forward legislation to limit the consequences of a “no deal” exit, ensuring public safety and protecting vital economic interests.

Any piece of legislation could be amended in this scenario to instruct the U.K. government to apply for an extension of the Article 50 negotiating period, call a second referendum — or even notify the EU of the U.K.’s intention to withdraw Article 50 altogether.

“What defines the Brexiteers is overestimation" — Cabinet minister

If Conservative MPs do not rally behind May’s plan en masse — improbable at best — May’s deal as it is will be rejected. Most parliamentarians believe it is far more likely that the government’s proposed Brexit package would then be considerably softened to win Labour support, or blocked altogether, rather than tweaked into a harder version of Brexit, as demanded by backbench Brexiteers.

One Cabinet minister, speaking anonymously, said the Brexiteers’ failure to trigger a leadership challenge this week has revealed their weakness.

“What defines the Brexiteers is overestimation,” he said. “They overestimate Britain’s strength in the negotiations. They overestimate their strength in the parliamentary party. They overestimate their strength in parliament in general. You can’t help feeling that bit more hopeful when you see their performance this week.”

British Empire

Almost 100 years after the Anglo-Irish Treaty established the Irish Free State inside the British Empire, Westminster politics is once again stuck in Ulster’s sectarian quagmire.

One leading Conservative Party Remainer said that because of the Irish backstop, the Brexit on offer is an impossible choice.

“The softer you make it, the more it calls into question why you are leaving at all,” he said. “But sacrificing Northern Ireland is not acceptable. I haven’t come across anyone who it is acceptable to. It’s a price none of us is willing to pay, which means you’re back to ‘What’s the point?’”

The Cabinet minister agreed: “Northern Ireland brings the Brexit trade-off into clearest focus, but the trade-off exists everywhere: The more you diverge, the more market access you lose.”

Those closer to the prime minister see it differently.

A second Cabinet minister, speaking on condition of anonymity, said the Brexiteers are “looking for a way back” and could eventually back the deal rather than risk no Brexit. They may hang their hopes on the “unicorn” option of a technological solution to the Irish border dilemma, the minister said. That is not ruled out by the divorce deal, but is dismissed by virtually everyone in London and Brussels as unfeasible.

In Belfast, there is growing panic among unionist hard-liners that given the risk of remaining in the EU, London will eventually choose to jettison Northern Ireland as an equal partner for the sake of greater freedom for Great Britain. Many unionists in Northern Ireland see a tariff and regulatory border between the nation and mainland Britain as amounting to abandonment.

If Britain is facing its own Irish Free State dilemma, it is does not bode well for the prime minister.

“Remind me what happened to Collins after he signed the treaty,” one MP said. Barely six months after the Anglo-Irish Treaty creating the Irish Free State was agreed in December 1921, Collins was assassinated by IRA gunmen. A civil war ensued, won by the pro-treaty forces.

Nobody is talking about violence, just threats to May’s career. But to this day, Ireland’s two main political parties remain the pro-treaty Fine Gael (now led by Irish leader Leo Varadkar) and the anti-treaty Fianna Fáil. Ireland eventually left the British Commonwealth in 1948, severing ties with the British crown.

Brexit divisions are still a long way from civil war, but whatever the outcome in the negotiations, there is a widespread belief among Tory MPs that May’s tenure as prime minister will not survive signing the divorce agreement.

That’s if she is even able to get that far.

“She’s no Collins,” said the DUP official. “He was a proper Republican. She doesn’t believe in Brexit.”