[4] That same year he joined the Prussian army as an officer, serving in the Queen's Fusilier Regiment (Schleswig-Holstein) No.86, under the command of von Kusserow.
The third battalion of the regiment was stationed in Sønderborg Castle in the area where two and a half decades earlier his father had served with distinction during the Franco-Prussian war.
[4] In November 1904 he took over command of the Twelfth Company in Mahenge, an inland location in the southern part of the German East Africa colony.
[7] The ensuing battle took the form of a siege in which von Hassel, his group of sixty native soldiers, a few hundred loyal tribesmen massively outnumbered, authorised the use of the defenders' two machine guns.
[4][7] During a lengthy period of home leave back in Germany, von Hassel met Emma Jebsen at a ball in Apenrade and married her in October 1906.
[11] It was partly in order to avoid Malaria[12] that Emma von Hassel spent most of her second pregnancy staying with her parents in Apenrade, where their first son, Friedrich, was born on 16 April 1910.
[12] In August 1914 the outbreak of the First World War heralded von Hassel's return to the Protection Force, now facing a large hostile British army.
Theodore and his wife attempted to start a new life together in nearby Glücksburg, which had now become the most northerly town in Germany, while the children lived for at least some of this period with relatives.
Von Hasel sought to create a new coffee plantation not on the site of his former farm in the north of the country, which had been expropriated, but near Mahenge where he had served during the early years of the century.
During the final months of his life he was joined in the enterprise by his son, Kai-Uwe von Hassel, who had trained for a career as an Agribusiness merchant during the preceding two years.