extraterritoriality or bridges and islands

As I was completing some of the bureaucratic tasks to settle into my new job, I found it a little ironic that the special vehicle registration office (Kfz-Zulassungsbehörde) for the Hessian state capital of Wiesbaden was located in a particularly contentious former exclave, the borough of Mainz-Kastel and probably the least allied location for a function peculiar to state authorities.
 I knew that there was a certain patriotic tug-of-war between the state capitals, facing each other on opposite sides of the Rhine, but I did not know about the details or history at first. In Roman times with the founding of the frontier fortifications at Mogonticum (Mainz, Mayence), the empire first crossed the Rhine at this point of land with a bridgehead established at Kastel, with first a wooden bridge in the year 11 BC and then a permanent stone structure in the year 71 AD.
The modern Theodor-Heuss Brücke was built in the same spot. A triumphal arch dedicated to the memory of Roman general Germanicus, who nonetheless was unable to penetrate far into Germany except via a narrow corridor of control hugging the Main and the Danube to just outside of Regensburg (Limes Germanicus, the German limits or frontier), stood here until probably the early Renaissance.

Mainz-Kastel existed as an enclave of the Rhineland-Palatinate (historically, itself a district, an exclave, of the non-contiguous Kingdom of Prussia, et al.) inside the municipality of Wiesbaden (the Grand Duchy of Hessen), with some notable interruptions, until the end of World War II. When mapping out the zones of occupation, and subsequent formation of the new federal states of Germany, the Allied forces decided that the border for the French and the American sectors would be drawn by the river, and it became administratively easier to realign Mainz-Kastel with Hessen. In the end, the historical feuding is mostly in fun, I think, and in keeping with the spirit of Karnival and the Fasching season.