Arnis was not always an island. It began as a narrow peninsula reaching into the Schlei, the long fjord-like inlet that cuts inland from the Baltic toward the town of Schleswig. In 1415, during the running quarrel over Schleswig between Denmark, the Counts of Schauenburg and Holstein, and the Hanseatic League, King Eric of Pomerania ordered two rough castles built to control the water — one at Schwonsburg, one on the height where the Arnis cemetery now sits. For defensive purposes the strip of land joining Arnis to the Angeln shore was cut through. The peninsula became an island, which is precisely why a duke could later hand it to a crowd of runaway skippers as a place nobody else wanted.

Rumohr responded to the exodus with a two-part strategy: he abandoned the plan to enserf the people still in Kappeln, and he made life difficult for the ones who had left. Of the sixty-five households that swore the oath, only about thirty ever built houses on the island. Then came the Scanian War and the long grinding conflicts between Holstein-Gottorp and Denmark. For several years Arnis buried more people than it baptised. What saved it was a tax break — Christian Albrecht's son Frederick IV offered eleven years' exemption to new settlers, and the town finally began to grow. The Schifferkirche, the skippers' church, had already gone up in 1673, a statement of permanence made when permanence was still a gamble. Its pulpit and its painting of the raising of the cross are the reasons art historians make the detour.

By the middle of the nineteenth century the gamble had paid off spectacularly. Arnis held more than a thousand people and, around 1860, some eighty-eight merchant sailing ships — a fleet that put this speck of an island in the same conversation as Flensburg and Eckernförde. The skippers organised. Together with their counterparts in Kappeln they founded the Combinierte Schiffergesellschaft, and when the state proved too broke to fix the silting mouth of the Schlei, the two communities financed the work themselves. The channel at Schleimünde, cut between 1794 and 1796, still follows what they built. Arnis also maintained a sailors' association of its own, unusual among German ports of its size, which tells you something about how seriously a town of a few hundred took its own maritime standing.

Shipbuilding outlasted the sailing fleet. The first recorded launch was a vessel named Hoffnung in 1728, and by the second half of the twentieth century four yards were operating on an island 800 metres long and 200 metres wide. Matthiesen & Paulsen shipped sailing yachts to the United States and Japan. Otto Eberhardt's yard employed more than forty people before it closed in 2013 — a genuine blow in a place this size — and reopened in 2017 under Henning and Peter Eberhardt. The surviving yards work mostly on maintenance and restoration now, wooden hulls coming out of the water each autumn to be scraped and painted. In a town with almost no other industry, they still matter.

Modern Arnis runs on yachts and visitors. A cable ferry has crossed the Schlei here since 1826, still hauling itself over on a wire to the Sundsacker side, and the crossing is short enough that drivers barely have time to switch off. Lange Strasse runs the spine of the town past the oldest dwelling, number 13, to the water. In July 2017 the town marked its 350th anniversary with a pageant, a church service and a theatre company performing Der Auszug nach Arnis — the exodus to Arnis — as another company had done at the two-hundredth anniversary a century and a half before. The story gets retold because it is a good one, and because a place this small has to keep insisting on the reason it exists.