[1] Kryssing became the first commander of the Free Corps Denmark in 1941 after being offered the position by the Danish Nazi Party.

By August 1941, Kryssing built up a battalion strength of 1,000 men with many of its members hailing from the ranks of the defeated Danish Army.

His replacement was a pro-National Socialist Dane, Christian Frederik von Schalburg who took over command on 13 February, 1942.

[3] In February 1944, Kryssing became commander of SS-Kampfgruppe Küste but resigned in June the same year because of personal issues.

In May 1945 Kryssing surrendered to the British forces and was handed over to the Danish Police in June 1946.