26 Draconis Campaign

The 26 Draconis Campaign was a lengthy conflict between the UNSC and several insurrectionist factions. While it was not the first battlefield of the Insurrection, the initial events of the campaign are widely acknowledged as the true spark that set the colonies ablaze.
==Callisto Incident==
On January 8, 2494, a lethal firefight erupted aboard a colonial freighter during a routine inspection by the Redoubtable-class corvette UNSC Callisto. The tragedy's thirty casualties counted 3 UNSC personnel and 27 colonial crewmembers. With no contraband found aboard the freighter, the skirmish was quickly labelled a massacre and inflamed anti-UEG sentiments across the system. 
On February 15, 2494, the UNSC Callisto was ambushed during another inspection: the inspection team was spaced, and the freighter's crew swarmed aboard the Callisto and massacred its entire crew. The hijacking of the Callisto, the murder of its 91 crew, and its subsequent employment by its hijackers against UNSC forces in the system provoked a massive backlash against colonial independence movements, though the news of the attack would not reach public audiences outside the system until late March. 
Action of 2 March 2494
In response to the hijacking, the UNSC deployed a battle group of three Sentinel light destroyers to the system, arriving on March 2. The battle group easily located the Callisto and pursued it towards an asteroid field; when the corvette hesitated at the threshold of the field, the battle group conducted a textbook attack, closing the distance and firing an overwhelming barrage of Ares anti-ship missiles at the Callisto. The corvette successfully executed a slingshot maneuver, placing a sizable asteroid between it and the battle group, after which a nuclear mine previously embedded in the asteroid was detonated. The blast destroyed all remaining anti-ship missiles in flight as well as the leading destroyer UNSC Buenos Aires. Both remaining destroyers suffered heavy damage with the UNSC Las Vegas seemingly disabled entirely. The sole remaining active destroyer, the UNSC Jericho, was immediately attacked by the Callisto and was subsequently disabled as well. The UNSC Las Vegas communicated its unconditional surrender to the Callisto and requested immediate medical assistance. The hijacked corvette maneuvered close to the Las Vegas and connected the ships by means of a cargo transfer tether. When the Callisto opened its cargo bay to initiate boarding operations, however, the Las Vegas unexpectedly fired an Ares anti-ship missile out of its cargo bay, through the tether, and into the interior of the corvette. The resulting explosion shattered the corvette internally, wrecking eleven decks and killing more than half its crew. Following the destruction of the connective tether, the Las Vegas maneuvered away from the corvette, achieved an ideal firing position, activated its missile targeting systems, and demanded the immediate surrender of the Callisto; the corvette, its reactors slagged and its hull barely keeping the ship together, was in no condition to resist and surrendered. Additional UNSC ships arrived hours later, enabling the recovery of the Jericho and Las Vegas, and taking the Callisto and its surviving crew into custody. 
The Action of 2 March 2494 was the bloodiest space engagement since the Interplanetary War, over three hundred years earlier. Eight hundred and sixty lives were lost during the engagement: 645 UNSC personnel (336 aboard the Buenos Aires, 211 on the Jericho, and 107 on the Las Vegas) and 206 insurgent crewmembers aboard the Callisto. The engagement was the first time the UNSC suffered the destruction of a naval warship at the hands of an opponent since March 2170. 
Taken with the earlier casualties of January 9 and February 15, the Callisto Incident resulted in 951 deaths, of which 748 were UNSC personnel, 206 were insurgents, and 27 were civilians. The destroyer Buenos Aires was lost with all hands and the Callisto was deemed irreparable and scrapped. Both the Jericho and the Las Vegas underwent extensive repairs and returned to service in 2495.
The Callisto Incident is commonly considered the start of the Insurrection, as most pro-independent movements prior to the incident were entirely political in nature and most post-Callisto movements embraced violence as a means to win independence. In April 2494, however, the widespread reaction to the Incident was overwhelmingly in favor of the UNSC. The deliberate and horrific murder of naval inspectors and crewmembers, the piracy involving the hijacked corvette, and the use of a nuclear weapon caused a sea change on Earth and throughout the Inner Colonies, coloring perceptions of insurrectionists for the next three decades and setting the political bodies of the UEG resolutely against pro-independence movements. In the Outer Colonies, however, the reaction to the Incident was more complicated. The January incident was already widely known and abhorred as a government massacre of innocents in which the perpetrators went unpunished. The subsequent hijacking of the Callisto and the murder of its crew was generally seen as a horrific act, both for the spacing deaths of inspectors and the massacre of crewmembers - many of which were themselves from Outer Colony families - and was more often than not condemned as an unjust act of vengeance than as a justified retaliation. Nevertheless, the February hijacking and the March 2 battle between the UNSC battle group and the Callisto demonstrated to would-be insurrectionists that the UNSC Navy was not invincible, that they could be beaten, and that violent resistance was not necessarily impossible.