The two ships were based in southern Italy during World War I to help ensure that the Austro-Hungarian Navy's surface fleet would be contained in the Adriatic.

The modifications included removing their center main battery turret and boring out the rest of the guns to 320 mm (12.6 in), strengthening their armor protection, installing new boilers and steam turbines, and lengthening their hulls.

Both ships escorted convoys to North Africa in late 1941, including Operation M42, where Andrea Doria saw action at the inconclusive First Battle of Sirte on 17 December.

Italy was permitted to retain both battleships after the war, and they alternated as fleet flagship until the early 1950s, when they were removed from active service.

Sources disagree regarding these guns' performance, but naval historian Giorgio Giorgerini says that they fired 452-kilogram (996 lb) armor-piercing (AP) projectiles at the rate of one round per minute and that they had a muzzle velocity of 840 m/s (2,800 ft/s), which gave a maximum range of 24,000 meters (26,000 yd).

For defense against torpedo boats, the ships carried nineteen 50-caliber 76 mm (3 in) guns; they could be mounted in 39 different positions, including on the turret roofs and upper decks.

[13] By the early 1930s, the Regia Marina had begun design work on the new Littorio-class battleships, but it recognized that they would not be complete for some time.

The ships now carried 2,530 long tons (2,570 t) of fuel oil, which provided them with a range of 4,000 nautical miles (7,400 km; 4,600 mi) at a speed of 18 knots (33 km/h; 21 mph).

[16] The existing underwater protection was replaced by the Pugliese system that consisted of a large cylinder surrounded by fuel oil or water that was intended to absorb the blast of a torpedo warhead.

[22] These modernizations have been criticized by some naval historians because these ships would eventually prove to be inferior to the British battleships they were meant to face (namely the Queen Elizabeth class, since by the time the decision to proceed was taken a war between Italy and the United Kingdom seemed more likely).

[20] Admiral Paolo Thaon di Revel, the Italian naval chief of staff, believed that Austro-Hungarian submarines and minelayers could operate effectively in the narrow waters of the Adriatic.

[27] Instead, Revel decided to implement a blockade at the relatively safer southern end of the Adriatic with the battle fleet, while smaller vessels, such as the MAS torpedo boats, conducted raids on Austro-Hungarian ships and installations.

[28] Andrea Doria and Duilio both cruised in the eastern Mediterranean after the war, and both were involved in postwar disputes over control of various cities.

Duilio was sent to provide a show of force during a dispute over control of İzmir in April 1919 and Andrea Doria assisted in the suppression of Gabriele D'Annunzio's seizure of Fiume in November 1920.

In January 1925, Andrea Doria visited Lisbon, Portugal, to represent Italy during the celebration marking the 400th anniversary of the death of explorer Vasco da Gama.

[29] Both Andrea Doria and Duilio went into drydock in the late 1930s for extensive modernizations; this work lasted until October and April 1940, respectively.

Duilio participated in a patrol intended to catch the British battleship HMS Valiant and a convoy bound for Malta, but neither target was found.

A force of twenty-one Fairey Swordfish torpedo-bombers, launched from HMS Illustrious, attacked the ships moored in the harbor.

[30][31] In February, she was attacked by the British Force H; several warships attempted to shell Duilio while she was in dock, but they scored no hits.

By the end of the year, both battleships were tasked with escorting convoys from Italy to North Africa to support the Italian and German forces fighting there.

During the latter, Andrea Doria and Giulio Cesare engaged British cruisers and destroyers in the First Battle of Sirte on the first day of the operation.

Convoy escort work continued into early 1942, but thereafter the fleet began to suffer from a severe shortage of fuel, which kept the ships in port for the next two years.

After learning of Duilio departure, however, British escorts scuttled the transport Rowallan Castle, previously disabled by German aircraft.

Italy was allowed to retain the two ships after the end of the war, and they alternated in the role of fleet flagship until 1953, when they were both removed from service.