Swap Meet

Many parts of the world have passed from one country to another, some a great deal of times.  But the most frequent sovereignty flip-flopper is certainly Pheasant Island along the border between France and Spain, which has been exchanged between the two at regularly scheduled intervals for the past 350 years.

In 1659, France and Spain had been at war for the past quarter century, and were ready to sign a peace treaty, but they needed an agreeable, neutral location in which to do so.  They met at Pheasant Island, a tiny piece of land in the middle of the Bisaboa River which forms part of their common border.  Obviously, the most important thing about a treaty ending decades of brutal war is who gets to claim credit for hosting the peace summit.  To avoid any imbalance on this key issue, it was agreed that the island would become a condominium - a piece of land over which two countries share control.

Usually, a condominium is simply jointly administered, with both countries simultaneously contributing to governing it.  However, in 1659, kings signing treaties were pretty much free to agree on whatever crazy scheme they wanted, and the countries instead decided that the island should swap back and forth every six months.  To understand how silly of a plan this is, it's important to know that, aside from a little monument marking the location of the treaty and a handful of trees, the island is empty.  It's too small to have any real significance or value.

Still, the two powers managed to find another use for it aside from signing treaties.  Dating back even before the 1659 treaty, the island had been used to exchange brides between the two royal families.  In 1615, Louis XIII of France and Phillip IV of Spain met on the island and exchanged sisters.  Louis married Philip's sister Anne, and Philip married Louis' sister Elisabeth.

Although the sister swaps have stopped now that France no longer has a monarchy, the shared custody of the island continues to the present day, making Pheasant Island the oldest condominium on the planet.  Since 1659 it has changed hands over 700 times.