After completing the third course at the Luftkriegsschule 2 (Air Warfare School) in Berlin, Trettner joined the Condor Legion and served as adjutant and IIa to Generalmajor Hugo Sperrle and Wolfram Freiherr von Richthofen during the Spanish Civil War.

Trettner then joined the Bundeswehr and was transferred to the Supreme Headquarters Allied Powers Europe (SHAPE) in Paris serving as commander of the logistics department until 15 September 1959.

Senior Communist official Albert Norden alleged that Trettner had been involved in the bombing of Guernica, during the Spanish Civil War.

[2] In 1965, former Spanish Republican Air Force commander Ignacio Hidalgo de Cisneros made a public statement criticizing Trettner's Bundeswehr appointment.

Heinrich Trettner was Catholic, and in 1978 he published an article, "The Holy See and Disarmament", in the German-language edition of the Vatican literary journal Communio, and a corresponding reply the next year.