TIVAT, June 7 -- The smallest country at the table was handed the biggest promise. Montenegro, a nation of fewer than a million people on the Adriatic coast, left a gathering of European leaders this week having been told it could be inside the European Union by 2028, the fastest accession timeline Brussels has dangled in front of anyone in years. The warmth was genuine. So was the calculation underneath it.

For thirteen years the bloc has admitted no one, a record of inaction that Germany's Friedrich Merz acknowledged in Tivat when he allowed that the long freeze showed shortcomings before pivoting to something plainer. We want you, the German chancellor said, and meant it as both an invitation and an apology.

What changed is not Monte...