Operation: TREBUCHET was the Unified Earth Government's military response to the numerous civilian uprisings in the Outer Colonies known as the Insurrection. Spanning over ten years and encompassing at least two star systems, Operation: TREBUCHET involved numerous Marine units and many vessels from the UNSC Navy. It is one of the largest and farthest reaching operations known to have been conducted by the UNSC in its long history.
Beginning in 2513 with a large uprising in the Eridanus system, Operation: TREBUCHET was intended to be a heavy handed fight to bring all dissenting Outer Colonies in line, effectively crushing all resistance to the Earth's authority in the Outer Colonies.
At first the aggressive response was widely accepted, however, as the fight dragged on and the theater of operations expanded, the continuing operation became extremely unpopular with the human civilian population. Operation: TREBUCHET eventually became the central conflict for Earth's premiere fighting force, the UNSC, only being ended with the massive genocide that followed with the Human-Covenant war.
UNSC forces begin their campaign to retake the planet, deploying a battlegroup of unknown size to engage the makeshift Insurrectionist defence fleet. Despite the UNSC's best attempts at removing the entrenched rebellion, ONI fails to discover the location of their base. This hollowed-out asteroid would remain a safe haven for Insurrectionists for another three decades.
Fighting breaks out in the Eridanus system first, particularly at Eridanus II and the space around it, as rebels supporting the system's breakaway from the UNSC build a small navy of civilian craft and attempt to take over the system. The disorganized Eridanus Rebels are eventually brought together by Colonel Robert Watts.
The UNSC responds harshly, sending a Fleet of Destroyers and Carriers to the system to combat the rebel fleet. The resulting battle pitted the UNSC against at least one hundred smaller rebel craft. The UNSC forces defeated their rebel counterparts with little difficulty. During the battle, the rebels were routed and the remnants of their fleet fled into the asteroid belt to Eridanus Secundus. They would eventually establish a fully functional asteroid base and began conducting raids again by 2525.
A ground campaign begins on Eridanus II, as units from the 9th Marine Expeditionary Force are sent to the planet in force, hoping to capture leaders of the movement and pacify the population. One of these units, the first Battalion of the 21st Marine Division commanded by Lieutenant Colonel Ponder, was involved in a disastrous mission to try and capture a rebel leader - the second in command - in Elysium city, which ended in the leader's family being killed and Ponder wounded and demoted.
Fighting continues, now in Epsilon Eridani. By this point, the war has taken over a million lives, the majority of which being civilians. A Marine Battalion is posted at the colony world of Tribute, commanded by Lieutenant Colonel Aboim and supported by the corvette UNSC Bum Rush and air support from Hornets. The battalion attempts to put a stop to rebel bomb makers. Two special warfare squads, led by Staff Sergeants Avery Johnson and Nolan Byrne, are involved in disastrous operation in June. Byrne's team is killed during a hostage situation at a Jim Dandy restaurant near Casbah city. Thirty-Eight civilians, three Marines, and two rebels are killed in the bombing. Byrne and Johnson are pulled from the system, and later sent to Harvest.
In the meantime, the Insurrectionists resort to bolder, more deadly tactics, such as the bombing of a civilian liner in orbit over Reach.
The last known operations involved the SPARTAN-II soldiers in 2525. A team of five SPARTAN-IIs were dispatched and to infiltrate the rebel base at Eridanus Secundus and capture Watts, the rebel leader. They succeeded in doing so, crippling but not destroying the rebel infrastructure in the Eridanus system. Shortly afterward, the Spartans were sent to halt a rebellion on Jericho VII.
The operation was abruptly concluded with the start of the Human-Covenant war after the First Battle of Harvest. Though the Insurrection continued for a long time, the rebel threat in the system was rapidly decreased when the genocidal Covenant destroyed the systems, glassing planets and killing every human they encountered. In the meantime, many of the rebels, seeing that the UNSC offered the only real protection against the overwhelming Covenant force, ceased to resist, and either submitted to the UNSC's control or hid themselves away from both UNSC and Covenant attack. Some groups of rebels survived free of UNSC control for as long as 2552. The majority simply continued their daily lives without interfering with the UNSC. Others even decided to offer dealings with the UNSC. However, most of these dealings were traps.
The rebel base in the Eridanus system was marked for further attack, but for some reason, likely due to the greater threat the Covenant posed, it was left alone and mostly forgotten until 2552 when Vice Admiral Danforth Whitcomb and a small crew including the AI Cortana and several Spartans (including all but one of the team that attacked the base) returned looking for repairs. After being convinced to help by a Covenant attack, the rebels repaired the Ascendant Justice-Gettysburg to operational status, but the two ships were too badly damaged to be fully repaired before the Covenant returned. The UNSC forces were forced to abandon the rebels and the base and the rebels have been presumably destroyed.