'QWZ' was a British and co-belligerent Italian naval special forces attack on ships and U-boats in the harbour of La Spezia in the German-aligned Salò Republic of northern Italy (20/21 June 1944).

Following the Allied armistice with Italy on 8/9 September 1943, the Italian navy’s 10a Flottiglia MAS commando frogman unit was disbanded, and the government headed by Maresciallo d’Italia Pietro Badoglio in Allied-held southern Italy declared war on Germany on 13 October of the same year and became a co-belligerent. Some men of the 10a Flottiglia MAS joined the Allied cause to fight against their erstwhile comrades who had joined the navy of the Repubblica Sociale Italiana, informally known as the Salò Republic, which was the revived Fascist state created by Benito Mussolini in northern Italy as a client of Germany.

In southern Italy, a new unit was formed in the co-belligerent Italian navy under the leadership of Capitano di Fregata Ernesto Forza, and this included a number of the original unit’s veterans, including Tenente Luigi Durand de la Penne, freshly freed from British prisoners of war camps, and well as a number of new volunteers. The new unit was named Mariassalto, but continued as an elite naval force for the planning and execution of maritime special operations.

The Mariassalto was formed at Taranto alongside the British frogman and 'Chariot' (human torpedo) force in the Mediterranean. Forza was more than happy to demonstrate to the British the high quality of Italian expertise in this capability, and the group was also enthusiastic to re-enter combat despite the fact that if any of its personnel had been caught they would almost certainly have been shot. However, the overwhelming supremacy of the Allied naval forces in the Mediterranean had left the new unit with few targets worthy of its skills.

In June 1944, though, there finally appeared an opportunity for the Mariassalto unit to re-enter combat in 'QWZ', a joint mission against targets in the harbour of La Spezia, the Salò Republic’s main naval base in north-western Italy. The attack was against the Italian heavy cruisers Bolzano and Gorizia, which had been seized by the Germans in 'Achse' (ii) after the Italian armistice. Bolzano had been damaged so badly that it was beyond the capabilities of the Germans and Italians in northern Italy to repair her, but she was nonetheless usable enough to be sunk as a blockship, and 'QWZ' was planned in an attempt to thwart a German plan to sink the two cruisers in a position in which they would block the harbour entrance. Another of the operation’s tasks was to attack U-boats in the harbour.

The plan was for British 'Chariot' human torpedoes to attack the cruisers and the Mariassalto’s frogmen to attack any U-boats in the harbour. On 25 June the destroyer Grecale of the Italian co-belligerent navy departed Bastia on the island of Corsica bound for La Spezia carrying three high-speed torpedo boats, the party of Italian frogmen including Durand De La Penne, and two British 'Chariots'. One of these last suffered a mechanical failure and was abandoned, though the other attacked and sank Bolzano. The frogmen were unsuccessful in their attack on the U-boat pens, however, but did manage to escape and join partisan groups.