Time as Phobos
The offspring of Ares, Phobos once served under his 'father' during Ma'chello's rebellion on the planet Dendred. Agents of Ma'chello, having swapped bodies with some of Phobos' Jaffa through Ma'chello's body switching device, gained access to the Ha'tak owned by Phobos. They proceeded to sabotage the Transportation rings on board, causing the beams they emitted to disintegrate any matter in their path rather than simply creating an energy stream which passed harmlessly between sets of rings, and let them be for some time. However, at a pre-arranged time, these agents activated their trap which cut right through the ship and destroyed it in a fiery explosion; they had done the same to the Ha'tak operated by Phobos' 'brother' Deimos. Through this, they thought they had killed both of Ares' 'sons', as well as losing him two of his precious Ha'tak's. (RPG: "Friends and Foes: Stargate Season Two")
They were wrong, however, as both of Ares' offspring somehow managed to survive the traps and indeed Phobos went on to become a System Lord in his own right. Sometime adter this, Phobos sought to usurp his father and replace him as the Greek god of war on Earth. Ares managed to crush his uprising, partially due to Mars' brother betraying him, and so Phobos fled to the Romans where he assumed the identity of his father, becoming Mars. The Romans worshiped him for a time before he was betrayed by some of his own who returned him to Greece to face Ares' wrath. Ares managed to crush Mars' spirit, who ended up begging for his life, and soon sent him through the Stargate to the traitors graveyard world of Kar'tek; the entire event was recorded on a Vo'carr as a warning to others who would oppose Ares.
It was on Kar'tek that he discovered the Volca, a species in which Goa'uld symbiotes could not survive, along with creatures known as Mar'kak. Because of the lack of suitable humanoid host bodies, Mars instead chose to build a cyborg body with a Mar'kak as its biological component; Mars would enter the creature and assume control of the cyborg body. It was through this method that Mars managed to rebuild his civilization, basing it on that of the Roman Empire, his Jaffa's symbiotes were put into further cyborg bodies in order to continue serving Mars.
In 1998, SG-1 found the vo'carr on another world where it was guarded by Apophis' Serpent Guard and, thinking that Mars would be long dead, chose to journey to Kar'tek which they had named TC9-173. They first sent a M.A.L.P. to scout ahead and soon saw a Volca on the camera, witnessing their unique biological ability to phase through solid objects. What they did not notice was Mars' Jaffa discovering the Volca and uncovering the chaapa'ai which had been lost to them for centuries. Once SG-1 stepped through themselves, they were met by the Jaffa of Mars who captured all but Daniel Jackson, Daniel being saved by Etahy, and brought them before Mars.
Mars was pleased to see the Tau'ri once more kneel before him, especially so soon after the lost Chaapa'ai had been found, and was curious if Teal'c was acting as a representative of Apophis, seeking an alliance against Ares. Upon hearing that the group "served" a Republican term, he assumed that Ares had fallen to his advisers. He then planned to take Jack O'Neill as his new host in order to enact his vengeance though Areleus, a member of his own senate, thought that the decision should be discussed in the senate proper.
It was at this point that SG-1 chose to attack, stealing staff weapons from Mars' Jaffa and firing them on their former holders. They did not succeed, however, and only managed to wound Mars' First Prime Augustus before their attack was brought to a halt. This act of violence earned Teal'c a place in the pits, and Areleus was sent with him for his defiance of Mars' will. During this time, Daniel had been taken to Ulysses, who he found to be another Goa'uld, by Etahy to seek his help. Ulysses presented him with a phase shifting device, less powerful than that of the Tollan, but would not help himself.
Back in the arena, Teal'c managed to best two of the "legends" who had been sent to fight him, the Minotaur and Medusa, but was bested in combat by the third, the Hydra. He would have been killed, as Mars ordered his death by lowering his thumb, but was saved due to the Jaffa on the senate rebelling against their "god" and attacking the Hydra before it could finish Teal'c.
This angered Mars, and he issued death warrants for every member of the senate as well as ordering his Jaffa to hunt down Teal'c and his rescuers so he could rip their larva out with his bare hands. At the same time, he started the preparations to transfer himself from the cyborg body into that of one of the Tau'ri. His Jaffa soon found the traitors, killing Jolian and taking the others as prisoners to face Mars' wrath. Mars chose to allow them to watch him take his new host in O'Neill before they were killed themselves; this act would have been completed if not for the timely arrival of Jackson, Etahy, and other Volca.
Arriving through the very walls of the building, the group dragged Jaffa backwards into said walls before letting them go; the Jaffa died instantly by coming back into phase inside solid objects. O'Neill took this moment to attack Mars with an energy sword but the attack was for naught as Mars raised an energy shield from his Kara kesh, instead repulsing O'Neill with one of the device's other functions. Although both Teal'c and Samantha Carter attempted to assault him physically, it was the actions of Etahy that stopped Mars; he simply phased his hand into the creature inside Mars' chest and ripped a hole into it.
This was thought to have killed Mars, but this was not true; When his First Prime arrived with Patrician, they removed the Goa'uld symbiote from Mars' former host and put it into the Mar'kek inside Patrician. Mars lived once more, informing his Prime that there was much to do. (SG1: "Fall of Rome 1")
Behind the scenes
Note that lowering your thumb was actually a sign to spare the person's life in the times of Ancient Rome and has simply been incorrectly portrayed in media.
References and notes