During the Easter Rising in Dublin in 1916, some republican leaders, including Patrick Pearse and Joseph Plunkett, contemplated giving the throne of an independent Ireland to Prince Joachim. Pearse and Plunkett thought that if the rising were successful and Germany won the First World War, an independent Ireland would be a monarchy with a German prince as king, like Romania and Bulgaria before it. The fact that Joachim did not speak English was also considered an advantage, as he might be more disposed to learning and promoting the use of the Irish language. In his memoirs, Desmond FitzGerald wrote:
"That would have certain advantages for us. It would mean that a movement for de-anglicisation would flow from the head of the state downwards, for what was English would be foreign to the head of the state. He would naturally turn to those who were more Irish and Gaelic, as to his friends, for the non-nationalist element in our country had shown themselves to be so bitterly anti-German.......For the first generation or so it would be an advantage, in view of our natural weakness, to have a ruler who linked us with a dominant European power, and thereafter, when we were better prepared to stand alone, or when it might be undesirable that our ruler should turn by personal choice to one power rather than be guided by what was most natural and beneficial for our country, the ruler of that time would have become completely Irish."
Ernest Blythe recalls that in January 1915 he heard Plunkett and Thomas MacDonagh express support for the idea at an Irish Volunteers meeting. Bulmer Hobson, secretary of the Volunteers, was among the attendees. No objections were made by anyone and Blythe himself said he found the idea "immensely attractive".
After his father's abdication, Joachim was unable to accept his new status as a commoner and fell into a deep depression, finally taking his own life 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.
After the divorce, Prince Karl married, morganatically, Luise Dora Hartmann on 9 November 1946. The childless couple divorced in 1959. Prince Karl's last marriage was to Eva Maria Herrera y Valdeavellano on 20 July 1959 in Lima, Peru. They were married until Prince Karl's death and had two daughters;
Alexandra Maria Augusta Juana Consuelo Prinzessin von Preussen
Désirée Anastasia Maria Benedicta Prinzessin von Preussen.