The Battle of the Tobin system occurred in 2000 between the forces of Apophis and Heru'ur. The battle was started after the Tau'ri and Tok'ra sabotaged a potential alliance between the two System Lords. The battle was rather one-sided and led to the death of Heru'ur and the absorption of his fleet into Apophis's. This ultimately resulted in Apophis becoming the most powerful Goa'uld until his fleet was destroyed a year later during the Battle of the Vorash system which led to the destruction of the Vorash system. (SG1: "Exodus")
The Tok'ra intercepted a transmission between Heru'ur and Apophis (then the two most powerful Goa'uld in the galaxy), who intended to meet. Though Heru'ur was ostensibly allied with the System Lords against Apophis, it seemed as though he was attempting to forge a deal with Apophis behind their backs. Such an alliance would prove too much for anyone in the galaxy to oppose, ultimately resulting in Apophis's complete domination over the entire galaxy. The Tok'ra representative Selmak formulated a plan with the Tau'ri, believing that their best bet would be to attempt to sabotage the meeting, causing Heru'ur and Apophis to war against each other, decimating both armies and returning the balance of power to the System Lords, which for the time being would be preferable to one, all-powerful Goa'uld.
After capturing Teal'c among the rebel Jaffa on Chulak, Heru'ur wished to use him as a bargaining chip with Apophis during their meeting to set up to parlay about combining forces. This meeting was set in a mine field surrounding a planet in the Tobin system, ostensibly to ensure that each of them would come alone. Jacob Carter/Selmak, however, recruited SG-1 to help him alter the programming on one of the Tobin mines so that it would home in on one of the motherships and cause each of the Goa'uld to think the other was attacking, so that they would destroy each other. (SG1: "The Serpent's Venom")
While Selmak, with the help of Dr. Daniel Jackson, worked on reprogramming the mines Teal'c was heavily tortured on Heru'ur's Ha'tak by Terok, a lesser Goa'uld. Both Rak'nor and Terok attempted to convince him that the Goa'uld are gods, telling him that they are only doing it for his own good. Teal'c was steadfast, however, and frustrated even Terok by his constancy.
SG-1 and Selmak arrived in the Tobin system and ringed one of the mines aboard. They begin attempting to reprogram the device, when suddenly, Apophis' mothership arrived, forcing them to cloak. At the same time problems with entering the code in the mine caused the device to arm, setting itself to explode within five minutes unless it could be deprogrammed.
Heru'ur and Apophis began the meeting, and Heru'ur offered the alliance as expected. Heru'ur also offered a gift, as a token of his goodwill - to SG-1's horror, that gift was Teal'c. Apophis accepted the offer, the alliance was sealed, and Heru'ur commanded Terok to send Teal'c over to Apophis. Terok went to get Teal'c, and decided to kill him himself, knowing that Apophis could (and would) simply resurrect him with a Sarcophagus. Rak'nor finally rebelled, as he could not watch someone who had suffered so bravely be killed, and attacked Terok with his own Rod of Anguish.
At the last minute, Major Samantha Carter realized they were making a mathematical error in the code they were attempting, which solved their problem. They succeeded in reprogramming the mine and having completed their part of the mission, they decided to attempt to rescue Teal'c by intercepting him during the ring transport between the motherships. However, they barely missed the teleportation, and since it was necessary to decloak to use the rings, they were discovered and fired upon. At this moment, the Tok'ra operative aboard Apophis' ship activated the beacon, attracting the mine to Apophis's ship.
However, it was not Teal'c who was transported to Apophis' ship, but Terok, still unconscious. Apophis was convinced that Heru'ur was mocking him, and then the mine impacted on Apophis's ship. In response, Apophis decloaked at least ten other Ha'taks and destroyed Heru'ur's Ha'tak. However, the mines activated, and began attacking Apophis's forces. Apophis's flagship was able to escape as the other Ha'taks took the brunt of the mines, which may have been Apophis's reason for bringing the cloaked fleet. As they observed the carnage Selmak's Tel'tak picked up a Death Glider which apparently managed to escape Heru'ur's ship before it exploded. Fearing it may be Heru'ur himself, they gave chase, sending a transmission asking which god the pilot worships. The pilot responded with a Tok'ra passphrase, and Jacob Carter revealed his identity to the pilot. The pilot was Rak'nor, who had Teal'c aboard, alive, though very weak. (SG1: "The Serpent's Venom")
Though Teal'c was rescued and Heru'ur killed, the mission was regarded as a failure. Apophis had survived, and his quick destruction of Heru'ur foiled the plan to start a war between the two Goa'uld. Moreover, Apophis would absorb the remnants of Heru'ur's fleet, rising to a position of strength allowing him to challenge the remaining System Lords. (SG1: "The Serpent's Venom")