From Halopedia, the Halo wiki
Some Diligence-class destroyers were operated by the Colonial Military Administration as peacekeeping ships. By 2525, the class had been retired for decades. However, with the dawn of the Human-Covenant War, some were brought back into service with the UNSC Navy and were retrofitted for combat against the Covenant.
During or after 2540, a Diligence-class destroyer, the UNSC Kronstadt, deployed a squad of Orbital Drop Shock Troopers and a SPARTAN-II commando, Cal-141, via SOEIV to the Covenant-occupied planet Heian. The commando team infiltrated a former alien settlement, where they engaged several Unggoy and a Jiralhanae chieftain before assassinating a Covenant Prophet. After the mission, which resulted in the deaths of the Spartan and one ODST, the vessel retrieved the survivors.
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