Spoiler warning: Plot and/or ending details follow.

The Tarsus IV Security Force was the civilian agency tasked with peacekeeping and law enforcement on the Federation colony world of Tarsus IV.

Prior to the crisis caused by the fungus attacking the colony's food supplies the security forces were led by Alexander Simmons. Once the crisis hit the colony the security force maintained martial law once it had been declared, and took steps to try to prevent the spread of the infection. After Adrian Kodos took over as Governor, some of the security forces helped him murder 4,000 colonists.

After Kodos was forced from office and Gisela Ribeiro was allowed to resume office, she and the Starfleet team had the difficult task of ferreting out the members of the forces still loyal to Kodos from those who had taken no part in the violence against the colonists. When Simmons was arrested Lieutenant Commander Philippa Georgiou and Lt. Commander Gabriel Lorca deliberately kept the news of the arrest limited to people they knew they could trust, fearing that moles inside the forces would inform Kodos of the arrest. (DSC novel: Drastic Measures)

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