After his father's abdication, Joachim was unable to accept his new status as a commoner and fell into a deep depression, finally committing suicide by gunshot on 18 July 1920 in Potsdam. One source reports that he had been in financial straits and suffered from "great mental depression". His own brother Prince Eitel Friedrich of Prussia commented that he suffered from "a fit of excessive dementia". Before his death, the couple had recently divorced. The direct causes are not really known to the public, only that there had been no previous report of marital troubles before the divorce was announced. Regardless of the reasons, this event may have also contributed to his depression.
Prince Karl married first, on 5 October 1940, Princess Henriette Hermine Wanda Ida Luise Schönaich-Carolath (25 November 1918 – 16 March 1972). They divorced 5 September 1946. They were the parents of three children:
Prince Friedrich Christian Ludwig of Prussia (3 September 1943 – 26 September 1943)
Prince Franz Friedrich Christian of Prussia (born 17 October 1944).
Prince Karl married secondly, morganatically, on 9 November 1946, Luise Dora Hartmann (5 September 1909 Hamburg, Germany – 23 April 1961 Hamburg, Germany). The childless couple divorced in 1959.
Prince Karl married lastly, on 20 July 1959 in Lima, Peru, Doña Eva Maria Herrera y Valdeavellano (10 June 1922 Lima, Peru – 6 March 1987 Lima, Peru). They were married until Prince Karl's death and had two daughters;
Alexandra Maria Augusta Juana Consuelo Prinzessin von Preussen (born 29 April 1960 Lima, Peru)
Désirée Anastasia Maria Benedicta Prinzessin von Preussen (born 13 July 1961 Lima, Peru).