[citation needed] Rintelen joined the Prussian Army and was promoted on 17 September 1910 as a fahnenjunker in the Grenadier-Regiment „König Friedrich Wilhelm IV.“ (1.

[citation needed] On 13 December 1915, he joined the staff of the 2nd Grenadier Regiment, becoming adjutant on 13 January 1916, and a company commander on 15 September 1917.

[citation needed] On 1 October 1936, Rintelen was posted to Rome as the German military attaché in Italy.

[citation needed] After Italy entered the Second World War in June 1940, he became the representative of the Oberkommando der Wehrmacht (OKW) to the Italian High Command (Comando Supremo).

Generalfeldmarschall Albert Kesselring was appointed OB Süd in December 1941, but initially controlled only the air forces in Sicily.

[2] Despite his warm relations with the Italians, Rintelen was under no illusions about the military prowess of Italy, which he felt was exaggerated by the Nazis due to their political affinity with fascism.

[2] In May 1943, he submitted a report on combat effectiveness of the Italian armed forces, concluding that they "have not up to now fulfilled the missions assigned them in this war, and have actually failed everywhere.

[3] When the Italian government changed in July 1943, Rintelen accepted the explanation of General Vittorio Ambrosio that it would have no effect on military operations and that Italy would remain in the war as an ally of Germany.

Adolf Hitler did not believe it, and used Rintelen, whom he considered an "Italophile", as cover while OKW prepared Fall Achse (Operation Axis) to disarm the Italian forces and occupy Italy.

[4] When he found out about it, Rintelen considered it a breach of faith with the Italians, and urged Kesselring to resign rather than implement it.

[citation needed] Rintelen worked for the US Army's Historical Division in 1946 and 1947, writing a monograph on German-Italian Cooperation.