In August 1915 the company came under the direction of Neapolitan entrepreneur Nicola Romeo, who converted the factory to produce military hardware for the Italian and Allied war efforts. Munitions, aircraft engines and other components, compressors and generators based on the company's existing car engines were produced in a vastly enlarged factory during the war. When the war was over, Romeo invested his war profits in acquiring locomotive and railways carriage plants in Saronno (Costruzioni Meccaniche di Saronno), Rome (Officine Meccaniche di Roma) and Naples (Officine Ferroviarie Meridionali), which were added to his A.L.F.A. ownership. Car production had not been considered at first, but resumed in 1919 since parts for the completion of 105 cars were still lying at the A.L.F.A. factory since 1915. In 1920, the name of the company was changed to Alfa Romeo with the Torpedo 20-30 HP becoming the first car to be badged as such.Their first success came in 1920 when Giuseppe Campari won at Mugello and continued with second place in the Targa Florio driven by Enzo Ferrari. Giuseppe Merosi continued as head designer, and the company continued to produce solid road cars as well as successful race cars (including the 40-60 HP and the RL Targa Florio).
In 1923 Vittorio Jano was lured away from Fiat, partly thanks to the persuasion of a young Alfa racing driver named Enzo Ferrari, to replace Merosi as chief designer at Alfa Romeo. The first Alfa Romeo under Jano was the P2 Grand Prix car, which won Alfa Romeo the inaugural world championship for Grand Prix cars in 1925. For Alfa road cars Jano developed a series of small-to-medium-displacement 4, 6, and 8 cylinder inline power plants based on the P2 unit that established the classic architecture of Alfa engines, with light alloy construction, hemispherical combustion chambers, centrally-located plugs, two rows of overhead valves per cylinder bank and dual overhead cams. Jano's designs proved to be both reliable and powerful.
Enzo Ferrari proved to be a better team manager than driver, and when the factory team was privatised, it then became Scuderia Ferrari. When Ferrari left Alfa Romeo, he went on to build his own cars. Tazio Nuvolari often drove for Alfa, winning many races prior to World War II.