In the 1920s and 1930s, the Italian Regia Marina (Royal Navy) built a series of heavy cruisers as part of a modernization program to use the tonnage allotted to Italy for cruisers by the international naval treaties of the period. A total of seven vessels were built to three designs: two s, four s, and , a modified version of the Trento design. The Regia Marina had initially intended to build six cruisers, which would operate as two three-ship divisions, but of the Zara class was used as the fleet flagship while the s were being rebuilt, so a seventh ship was ordered. The seven ships represented disagreements within the naval high command, with one faction favoring high speed at the expense of armor protection (which resulted in the Trentos and Bolzano) and another that preferred heavier armor in exchange for a reduced top speed (which produced the Zaras). All of the ships were officially built to the limitations imposed by the Washington Naval Treaty, namely a standard displacement of and an armament of main guns, though they all exceeded the displacement limit.
All seven ships saw extensive service in the Mediterranean Sea during World War II, where they repeatedly engaged elements of the British Royal Navy. Italian heavy cruisers took part in the Battles of Calabria, Taranto, Cape Spartivento, Cape Matapan, First and Second Sirte between 1940 and 1943. Three of the Zaras—, , and Pola—were sunk in a close-range night action with British battleships at the Battle of Cape Matapan in 1941, Trento was torpedoed and sunk by British forces in June 1942, and Bolzano was badly damaged by a British submarine in August 1942, which left her out of action for the rest of the war. United States heavy bombers raided La Maddalena in April 1943, sinking Trieste and badly damaging Gorizia; the latter vessel and Bolzano were later attacked by Italian commandos while Italy was under German occupation in 1944 to prevent the Germans from repairing and reactivating them. They were both raised and scrapped after the war.
Trento class
The Trento-class cruisers were ordered in 1924 in response to the French cruisers of the , begun that year. The Italian naval high command requested that the proposed cruisers have a very high top speed of ; the 1922 Washington Naval Treaty limited the standard displacement of new cruisers to and their main armament to guns. To keep the design under the limit while achieving the speeds requested by the high command, the design staff was forced to drastically reduce the scale of armor protection. Their weak armor led officers in the Regia Marina to demand better-protected vessels for the next class of cruiser to be built. Ironically, the high speeds attained by Trento and Trieste could only be reached on very light displacements and in service, they could only manage around , the same speed as the sturdier Zaras.
After Italy entered World War II in June 1940, Trento and Trieste saw extensive action against British forces in the Mediterranean Sea, including at the battles of Calabria, Cape Spartivento, and Cape Matapan in July and November 1940 and March 1941, respectively. In November 1941, Trieste was torpedoed by the submarine ; she spent most of the next year under repair. In the meantime, Trento was also present during the inconclusive First and Second Battles of Sirte. The Italian fleet sortied to attack the British Operation Harpoon convoy in June 1942, and while on the way, Trento was torpedoed twice, first by a Bristol Beaufighter torpedo bomber and then sunk by the submarine with very heavy loss of life on 15 June. Trieste was moved to La Maddalena, Sardinia, where she was later sunk by United States heavy bombers on 10 April 1943.
Zara class
While the Trentos were still under construction, elements of the Italian naval command began to doubt the effectiveness of the new vessels, which sacrificed armor protection in favor of very high speeds. They advocated a more balanced design that would incorporate more comprehensive armor, while retaining the battery of eight 203 mm guns and a speed of at least . To accomplish these goals, the high command permitted the designers to exceed the 10,000-ton limit, and instructed them to adopt as many weight-saving changes as possible. These included using a light weight propulsion system, removing torpedo tubes from the design, and abandoning the flush deck of the Trentos in favor of a stepped down main deck. The heavy armor of the Zaras made them the best-protected cruisers built by any navy until the United States' , laid down in 1945.