During the invasion of the Phoenix in 3721 A.D, the Unified Terran Nations' Federal Navy conducted an overwhelming military campaign against Aquarius Cluster's second, less well known superpower: The Phoenix Republic. The invasion was intended to remove the Republic's viability as a military threat entirely, and would result in the annexation of the sector.
While the Military campaign against the Republic's primary fleet was fairly quick: Federal Navy Fleets possessing numerical, technological and in many cases superior skills and training, the conflict would transition to a lengthy counter-insurgency operation by the Federation that would last well into the early 4th Millenium, right up until the Fall of the UTN in 4040 A.D.
The Phoenix Republic's primary battle-fleet, consisting mainly of CEG-Era warships and a mixture of natively-designed vessels was defeated within the first few cycles of the invasion. Republic Naval commanders soon realised they couldn't face the UTN's military head-on: so they switched to guerrilla warfare by state-funding local mercenary and pirate groups to attack the opposing UTN Fleets. This had limited success in open-combat, but ultimately would cause the conflict to drag on for centuries as the UTN tried to restore order to the sector.
The Phoenix Republic's Navy wasn't entirely destroyed in the opening cycles of the campaign. In fact, two vessels of the Fleet earned an infamous reputation by UTN Naval Commanders: known as the "Sisters", the CEG-Era Hammer-class Heavy Cruisers, Vysokiy and Molotok, both detached from the main force of the Republic's fleet, managed to wreak havoc on Federal Navy operations for at least a decade before being finally tracked down and destroyed.
UTN Naval Commanders attribute the success of the sisterships to a mixture of unconventional, 'hit and run' tactics and CEG espionage of UTN fleet movements in the early cycles of the invasion. Either way, the two Heavy Cruisers disabled or knocked out at least three Vengeance-class Heavy Cruisers, seven Farlease-class Light Cruisers and heavily damaged an Endeavour-class Battlecruiser and at least a dozen more vessels in their operational history, earning them a 'Bismarck'-esque reputation.