Bartolomeo Colleoni served in the Mediterranean until November 1938, when she sailed to relieve Raimondo Montecuccoli in the Far East. She arrived off Shanghai on December 23 1938, and remained there until the outbreak of war between Britain and France and Germany. On 1 October, having turned over command in the Far East to the sloop Lepanto, the cruiser returned home, where she arrived on 28 October.
Colleoni formed the 2nd Cruiser Division in the 2nd Squadron together with Giovanni dalle Bande Nere. Her first operation was a minelaying sortie on 10 June 1940 in the Sicilian Channel, followed by troop convoy cover duties between Naples and Tripoli in July.
On 17 July the ship sailed from Tripoli, accompanied by Bande Nere and bound for Leros in the Aegean, where British activities in Greek waters were causing concern. In the early hours of 19 July, while off Cape Spada (Crete), the Italian squadron, having been reported by RAF aircraft the previous day, was intercepted by the Australian light cruiser HMAS Sydney and five destroyers. During the ensuing engagement Colleoni eventually received a shell hit to the engine room from Sydney, which immobilised her and left her an easy target for the destroyers' torpedoes.
She sank with the loss of 121 sailors.
Probably the major historical interest of this battle was that it revealed the inflated nature of Italian official trial stats. On paper (on trial) the Colleoni had been listed with a maximum speed of over 40+ knots and should never have been overhauled by the 32-knot Sydney. The truth of the matter is that the Italian Navy had been in the habit of running speed trials not only at light-weight (without ammunition and full fuel loaded) but also without much of their designed armament.