In the border river Bidasoa, between Hendaye in France and Irun in Spain, lies an island of roughly 200 by 40 metres. It is uninhabited, closed to the public most of the time, and carries a monument. Twice a year it changes nationality.
From 1 February to 31 July Spain administers Pheasant Island, from 1 August to 31 January France does. The island is not divided, and the two states do not govern it jointly either. They take turns. Formally this is a condominium, and it is held to be the smallest in the world.
Why here of all places
The island has a history far larger than its surface. Because it lay between two empires and belonged clearly to neither, it became for centuries the place for negotiations that neither side wanted to hold on the other's ground. Whoever crossed over set foot on no foreign soil and so conceded nothing.
In 1659 the Peace of the Pyrenees was signed on the island. It ended a long conflict between Spain and France and made the Pyrenees the border between the two empires. A year later Louis XIV and the Spanish infanta Maria Theresa met here; their marriage was part of the same settlement. On both occasions the place was half the message: ground on which nobody was the host.
In 1856 the Treaty of Bayonne settled the border in detail, including its course through the river. That is where the half-yearly handover is written down. Since then the naval commanders of San Sebastián and Bayonne have administered the island in turn. Visitors need permission to set foot on it.
A place that belonged to nobody became, for that very reason, the place where agreement was possible.
— TerraTally · Borders desk
The handover has worked for more than a century and a half without incident. Part of the reason is that little is at stake. The island is uninhabited and economically worthless, and the river keeps eating at it: it is smaller today than it was when the treaty was signed. A quarrel over it would be a quarrel over a shrinking sandbank in an estuary that both sides can see from the shore.
That is precisely what makes it a counter-example. Territorial arrangements rarely fail because the law cannot solve them. They fail because one side wants more than the other can give up. Where there is nothing to win, even a cumbersome procedure can be kept up for two hundred years. The procedure holds not because it was cleverly designed, but because nobody has a reason to break it.
That the island is looked after at all is the real point. Two administrations keep records on a piece of land that pays no tax, houses nobody and is slowly disappearing. They do it because a treaty says so, and because a treaty kept is worth more than the ground it covers.
So twice a year somebody makes the crossing and responsibility changes hands. No border post, no flag, no ceremony. Just a date in the calendar that two states have honoured since 1856.