The three Wiesbaden suburbs of Amöneburg, Kastel and Kostheim, AKK for short, were incorporated into Mainz on 01.04.1908 (Kastel and Amöneburg) and 01.01.1913 (Kostheim). After the Second World War, the victorious Allied powers designated the Rhine as the natural border between their occupation zones in accordance with the Zone Protocol. By decree of the American officer Leroy Cowart, the three AKK suburbs were incorporated into WI on August 10, 1945. Since the founding of the federal states in 1945 and 1946, they have belonged to Hesse, still have "Mainz-" in their district name and are treated separately in the budget of the city of Wiesbaden.

All efforts to resolve the so-called AKK conflict in the sense of reincorporating the AKK suburbs into Mainz were unsuccessful. An official survey of citizens in 1986 showed a majority of 58.3% in favor of the status quo. A survey in 2006 did not produce a different result either. Attempts to reach a change as part of an agreement between the states of Hesse and Rhineland-Palatinate also failed. The state border, which is now over 60 years old, represents an almost insurmountable obstacle to restitution, as a change in state affiliation would only be possible under Article 29 of the Basic Law. By amending this article with regard to the number of inhabitants affected (the limit is now 50,000), it would be possible today to change the national affiliation only by state treaty and without a prior obligatory referendum. However, this remained just a thought experiment, as the Hessian side had always signaled its rejection. It is worth noting that the loss of three other districts on the right bank of the Rhine in Mainz in 1945, Bischofsheim, Ginsheim and Gustavsburg, which were not transferred to Wiesbaden but to the district of Groß-Gerau, did not provoke any comparable reactions from the Mainz population.