In January 1920, Visconti-Prasca was assigned to Germany as a member of the Inter-Allied Commission for Upper Silesia. In 1921, he joined the War Ministry and in 1922 the Army Council. Appointed military attaché to Yugoslavia in March 1924, he was recalled in 1930 when Yugoslav leaders alleged he was spying.
Visconti-Prasca then joined the General Staff. After a period commanding the 36th Regiment, he was appointed to the Bologna army headquarters. In October 1933, he was assigned to the office of the army chief of staff but was dismissed from that post by the chief of the General Staff, Pietro Badoglio, because he was suspected of having disclosed a secret agreement between Badoglio and French General Maurice Gamelin.
From December 1934, Visconti-Prasca commanded Italian troops in the Saar as part of the supervising League of Nations administration there. In January 1935, he was promoted to brigadier general and became an honorary aide-de-camp to King Victor Emmanuel III. That September, he assumed command of the 2nd Cosseria Infantry Brigade. In July 1937, Visconti-Prasca became military attaché in Paris, and in September of the same year, he was promoted to division general. In December, he was recalled to Italy; then he was assigned to the General Staff in March 1939. In June, Visconti-Prasca was appointed to command XXVI Corps in Albania, and the following month, he was advanced to corps general. That December, he commanded Eleventh Army in Albania and worked on the plan for the Italian invasion of Greece that called for four binary divisions attacking along a 60-mile front. Visconti-Prasca was absolutely convinced Italy would be victorious, a view that he trumpeted to Italian dictator Benito Mussolini and the General Staff.
The Italian invasion of Greece in October 1940 quickly turned into a disaster. Relieved of his command on 8 November, Visconti-Prasca was replaced by General Ubaldo Soddu. In December, he was forced to retire altogether from the military; he unsuccessfully appealed his retirement several times. Following the armistice with the Allies, he was deported to Germany. Freed there at the end of the war, Visconti-Prasca defended his actions in a book, Io ho aggredito la Grecia (1947), and several articles. He died in Monte Porzio Catone, near Rome, on 25 February 1961.
Bigini, Antonello, and Alessandro Gionfrida. Lo stato maggiore generale tra le due guerre. Rome: Ufficio Storico SME, 1997.; Italian Army Historical Office Archive, Raccolta Biografie, bundle 67. Rome.; Knox, McGregor. Mussolini Unleashed, 1939–1941: Politics and Strategy in Fascist Italy's Last War. Cambridge: Cambridge University Press, 1982.; Montanari, Mario. La campagna di Grecia. 4 vols. Rome: Ufficio Storico SME, 1980.; Visconti-Prasca, Sebastiano. Io ho aggredito la Grecia. Milan, Italy: Rizzoli, 1947.