French authorities have admitted that the mastermind behind the Paris massacre, Abdelhamid Abaaoud, who was killed in a shootout with police in the Paris suburb of Saint Denis on Wednesday, had re-entered France by way of the Greek migrant route. France’s interior minister said that French intelligence, which kept Abaaoud on a terror watch list, had no idea he had re-entered the country. A senior member of France’s spy agency told reporters that “Schengen is a sieve. A guy with a CV and history like this, wherever he turned up in Schengen, he should have sparked a red flag.” As many as two other gunmen participating in Friday’s attacks had also used the migrant path.Politically, this will definitely inflame the refugee debates in both Europe and the U.S. Policy-wise, though, its immediate impact will likely be felt on the Schengen Zone. EU interior ministers are set to agree today at an emergency meeting to French demands that Schengen’s external borders be strengthened, and that additional checks on EU citizens traveling within the area be implemented. But strengthening the external borders of the Schengen zone, which includes 30,000 miles of coastline and about 6,000 miles of land, is a daunting and costly task in practice. Any announcements are likely to be big on rhetoric but short on implementation details.One alternative solution being mooted: The creation of a mini-Schengen zone, comprising only Germany, Austria, Belgium, the Netherlands, and Luxembourg. The plan has apparently long been favored by the Netherlands, but has not gotten any traction until the recent strains on Schengen appeared. The plan conspicuously does not include France, and is still opposed by Germany. Discussions of the plan have taken place, however, and Germany did participate.These discussions bear a striking resemblance to plans put forward to “save” the euro by reducing it to a much more evenly balanced, economically suitable “northern euro” zone. Like those plans, it’s unlikely to be acted upon—the European elite has too much invested in the idea of “ever-closer Union” to roll the EU project backward. But in both cases, some of the Eurocrats seem to have learned that their initial, good-intentions-conquer-all idealism was overblown, and that they should have started with more modest goals. As the EU confronts multiple crises, we hope at the very least that this lesson will be remembered going forward.