Operation Crassus was a military and security operation conducted between February 3279 and April 3281. The operation was a response to the Far Rim Insurrection. In early 3279, the remnant forces of several defeated alien states and empires banded together in a loose alliance and instigated attacks on military and State Security installations and personnel.

The National Security Committee advised the deployment of a joint task force to put down the revolt as the local defence forces were overwhelmed by the end of January. On 3 February, Grand Admiral Charles Wilkins Taylor was appointed commander of Task Force Crassus, a combined Spacy and Security Forces unit. Operation Crassus, named after an ancient Roman general responsible for putting down a slave revolt in the 1st century BCE, was initiated upon the arrival of the Task Force to the Outer Rim. It began with a coordinated bombardment of worlds which were known to have sheltered remnant forces, as indicated by military intelligence. The primary bombardment operations continued for 18 months, and were followed by mop-up ground engagements by State Security armoured units. Additional space naval operations were conducted in parallel, hunting and destroying deep space bases from which rebels continued to launch ambush strikes.

Panzerkorps Kessar was redeployed in August 3280 to the Outer Colonies to handle the crisis on Keystone. The other Security Forces units were bolstered by Galactic Marine expeditionary forces during the second stage of Operation Crassus, lasting from August 3280 to April 3281, wherein the rebel forces were eradicated. Combat operations ceased on 16 April 3281, and legal process began against surrendering alien combatants and their human sympathisers. Over a hundred million were executed for treason.

During the combat phases of the Outer Rim Insurrection almost half a billion human civilians were killed by alien rebels, largely in nuclear attacks by rebel terrorists, and almost half of them in the first few months. A classified number of alien non-combatants were killed during the orbital bombardments. By the end of combat operations, around six million CGU servicemembers had been killed in action.

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