"Black December" Incident
Supposedly instigated by a radical Terran independence group called Black December, the "Black December" Incident was actually staged by ROM to discredit and suppress any such anti-ComStar activities on Terra.
By the start of Andrea Marteen's reign as Primus of ComStar many citizens on Terra had begun to chafe under the Blessed Order's rule of the world as benevolent dictators. While ComStar had physically rebuilt and restored Terra, they had also unintentionally restored many Terrans belief in the glorious days of the Terran Hegemony, with a number of militant political groups advocating converting the Blessed Order from ruler to mere tool of a reborn Terran empire. The frightened Order reacted poorly, using ROM to round up the worst troublemakers and prompting the radicals escalate their efforts to include terrorist-style bombings of ComStar re-education camps and other facilities.
Less extreme than her predecessor Raymond Karpov, Marteen adopted a carrot and stick approach. On one hand she publicly made speeches supporting a "Terra First" policy and construction of twelve massive arcologies, creating jobs for millions and improving the standard of living for millions more.  Secretly however, on the 3rd of March 2877 Marteen ordered Precentor ROM Joleen Weber to use whatever means necessary to suppress the Terran independence moment.  
In the summer of 2877 ROM agents exploded a bomb in the Euro-English Channel Tunnel which killed thousands of civilians instantly. Messages supposedly from members of Black December claimed responsibility for the bombing which they stated was meant to target the visiting pro-Marteen Precentor New Earth Jerrold Komandi, who narrowly avoided his date with destiny due to an unexpected and perfectly timed traffic jam.
When ComStar sent huge number of rescue teams and relief equipment to save as many people as possible, in full view of the media of the Inner Sphere, Black December lost whatever support it might have once enjoyed. Turning the bombing into a propaganda bonanza, Marteen called for the members of the group to be caught and put on trial. ROM operatives allegedly discovered the hideout of Black December three months later, killing all its members in a shoot-out that erupted as they tried to arrest them. The news of the destruction of the group, coming on the heels of the construction of the first of the projected megacomplexes, was widely welcomed by the public, dissatisfaction with ComStar having all but evaporated.
While some in both the First Circuit and outside the Order questioned the timing of the event, just when ComStar began to suffer the most heat from the various Terran independence moments, accusing Primus Marteen of complicity in the incident, any evidence was safely sealed away, only coming to light after the ComStar Schism.   
- Jihad Hot Spots: Terra, pp. 154-155, "Terra under ComStar"
- ComStar, pp. 33-34, "Slow Change"
- ComStar, p. 34, "Behind The Scenes"
- Intelligence Operations Handbook, pp. 104-105, "Witch Hunts"