[1][2] He participated in the First World War, receiving a Bronze Medal of Military Valour for an action on the Isonzo Front in September 1916 and being promoted to major on June 16, 1917.

[5][6] After Italy entered World War II on June 10, 1940, he was promoted to major general on January 1, 1941, and given command of the 24th Infantry Division Pinerolo.

[7][8][9] On June 26, 1942, De Stefanis replaced General Ettore Baldassarre, killed by an air strike near Mersa Matruh, as the commander of the XX Motorized Corps, of which "Ariete" was part.

With the corps reduced to brigade size by the losses suffered at El Alamein, having lost all manoeuvring units, De Stefanis was replaced by General Taddeo Orlando on February 4, 1943 and repatriated.

On the following night, as German forces implemented Operation Achse, he was among the high-ranking officers who fled Rome for Ortona following Marshal Pietro Badoglio and the royal family, reaching Brindisi with the corvette Baionetta.

In June 1944 De Stefanis became head of Delegation A of the General Staff, supervising the organization of Italian troops that would fight alongside the Allies as the Combat groups.