[3] Rosengren served as a teacher at the Royal Swedish Army Staff College from 1943 to 1945 and he was appointed captain in the Life Regiment Grenadiers (I 3) in 1945.

From 1957 to 1958, Rosengren served as Chief of Staff and Deputy Commander of the United Nations Emergency Force (UNEF) in Gaza.

In 1961, Rosengren was appointed head of the newly created Swedish Armed Forces Staff College (MHS).

[2][3] He was also a board member of the Military Literature Association (Militärlitteraturföreningen) from 1947 and of Hörsta förlag AB from 1958 to 1961.

[2] In 1940, Rosengren married Mai-Gret Legerth (born 1918), the daughter of Ragnar Jansson and Anne (née Du Rietz).