This week’s row over the Irish border marks a significant moment in the progress of Brexit: It is the first time since the referendum that the British political system has been forced to confront a genuine trade-off.

Until now, the big Brexit arguments—over the Brexit bill and the rights of European Union citizens in the U.K.—haven’t required the government to make choices so much as to face realities. But the Irish-border issue has obliged it to confront for the first time the possibility that the EU has been right all along: That it may not be possible for the U.K. to have its cake and eat it.

Under pressure from the EU, the U.K. was being asked to choose between accepting a hard border on the island of Ireland, a border in the Irish Sea, or remaining in the EU customs union. Prime Minister Theresa May hoped to fudge the issue with diplomatic language that committed the U.K. to maintaining regulatory “alignment” with the EU while at the same time ruling out any deal that would treat Northern Ireland differently from the rest of the U.K.

As a result, the fate of Brexit now hinges on the definition of the word “alignment.”

According to Irish Prime Minister Leo Varadkar, “alignment” means exactly the same as “no divergence,” which was the wording in an earlier draft. From an Irish perspective, it could hardly mean anything else—since any deviation by the U.K. from EU customs union and single-market rules would inevitably lead to the need for border checks. A partial alignment of single-market rules only in those areas critical to the working of the Good Friday Agreement would still result in a hard border, albeit softer than it might have been.