Mar Sara Colonial Militia was the "home-grown" self-defense and police force of Mar Sara.
Under the rule of the Terran Confederacy, the militia frequently used antiquated military equipment. It operated mainly independently from the Confederate military. The militia discouraged the use of stimpacks due to addiction.
Shortly afterwards, soldiers at Backwater Station came under attack by the zerg. Alpha Squadron commander Edmund Duke knew of their predicament but made little to no effort to rescue them. Raynor headed out by himself with the Magistrate sending in a militia force, the former personally leading his troops to the Station, rescuing the soldiers and destroying a nearby infested command center.
An angry Duke arrested Raynor and many standing militias when it became clear the the militias were willing to face the zerg while the Confederates were not. The Confederates refused to engage the zerg and further hindered the militia's ability to defend the planet by 'liberating' their hardware. The only source of military relief was the Sons of Korhal.
Mar Sara City came under attack by the zerg. Arcturus Mengsk, leader of the Sons of Korhal, made a deal with the Colonial Magistrate; he would provide transport craft to evacuate the colonists, provided that the militia could hold out that long. Meanwhile the Sons of Korhal rescued Raynor and his men from a Confederate prison ship.
The Magistrate, Raynor and other militia members joined with the Sons of Korhal, which evacuated the planet. The organization ceased to exist due to its fusion with the Sons of Korhal, as well as the destruction of Mar Sara by the protoss. Raynor came to lead a Sons of Korhal unit called "Raynor's Rangers". Later, many of its members joined the breakaway faction of Raynor's Raiders.
With the resettlement of Mar Sara and the Terran Dominion's presence on it, it appears that another colonial militia was founded on Mar Sara. When the zerg once again invaded the planet in 2504, history repeated itself, the militia helping defend Riksville long enough for evacuation by the Hyperion.
In StarCraft, the faction colour is blue with the exception of Backwater Station personnel, whose colour is purple.