The Decima Flottiglia MAS (Decima Flottiglia Mezzi d'Assalto, also known as La Decima or Xª MAS) (Italian for “10th Assault Vehicle Flotilla”) was an Italian commando frogman unit of the Regia Marina (Italian Royal Navy) created during the Fascist regime.
The acronym MAS also refers to various light torpedo boats used by the Regia Marina during World War I and World War II.
Decima MAS was active during the Battle of the Mediterranean and took part in a number of daring raids on Allied shipping. These operations involved surface speedboats (such as the Sinking of HMS York), manned torpedoes (the Raid on Alexandria) and Gamma Frogmen (against Gibraltar). During the campaign Decima MAS took part in more than a dozen operations which sank or damaged five warships (totalling 72,000 Gross Register Tonnage) and 20 merchant ships (totalling 130,000 GRT).
In 1943, after Benito Mussolini was ousted, Italy left the Tripartite Pact and joined the Allies. Some of the Xª MAS men who were stationed in German-occupied northern Italy enlisted to fight for Mussolini’s newly formed Italian Social Republic (Repubblica Sociale Italiana or RSI) and retained the unit title, but were primarily employed as an anti-partisan force operating on land. Other Xª MAS men in southern Italy or other Allied-occupied areas joined the Italian Co-Belligerent Navy as part of the Mariassalto (Naval Assault) unit.