The Battle of Tegalus was the culmination of an attempt by Stargate Command to disable a satellite weapons platform provided to the Rand Protectorate by a Prior of the Ori. Tragically, the battle ultimately resulted in the destruction of the Earth ship, Prometheus, a move that not only destroyed the ship but also killed thirty-nine of the ship's personnel including her commanding officer, Colonel Lionel Pendergast.
Some months after losing contact with the Rand Protectorate, Stargate Command was visited by Jared Kane, who had traveled to Earth without his superiors' knowledge in order to seek aid in preventing the Rand leadership from using the weapon to destroy the Caledonian Federation. He subsequently returned to Tegalus with Dr. Daniel Jackson in order to seek a diplomatic alternative; however the threat of the Ori satellite was deemed sufficient that Prometheus was dispatched to destroy it if negotiations failed.
The negotiations never took place, as both Jackson and Kane were immediately imprisoned upon their arrival on Tegalus; the Rand leadership was convinced that Kane was a Caledonian agent, and that the SGC had now allied with the Caledonians as well. Jackson's attempts to convince them otherwise were unsuccessful. (SG1: "Ethon")
Immediately after the Prometheus came out of hyperspace, Colonel Lionel Pendergast attempted to beam Dr. Daniel Jackson on board the ship, but as he had been separated from his locator beacon, this attempt failed. The ship's weapons then targeted the Ori satellite, but Pendergast deferred to SG-1 as an attack would likely end any chance of negotiating for Jackson's release.
On the surface, President Nadal showed no such hesitation, ordering an immediate attack on the ship. When the weapon began to power up, the Prometheus opened fire with its missiles; but the weapons impacted a shield, causing no damage to the satellite.
The satellite then fired a beam weapon that penetrated the Prometheus' shield, severely damaging the ship.
As Lt. Colonel Samantha Carter and Captain Kevin Marks attempted to repair the ship's engines, Lt. Colonel Cameron Mitchell and Teal'c along with the ship's the F-302 fighter-interceptors left the hanger bay, attempting to overwhelm the satellite's shields. Despite repeated missile strikes, however, the satellite was unaffected due to the shields. A second shot left Prometheus crippled, and the engines proved too badly damaged to be repaired in time.
Pendergast later ordered the crew to evacuate, having had them beamed to the surface of Tegalus within Caledonian territory. In order to buy time, he contacted the Rand command center and offered to surrender. Nadal refused, however, and ordered the ship destroyed. A third and final shot from the satellite split Prometheus at the neck, and both sections of the ship subsequently exploded with Pendergast dying as well. Despite the tragedy, two-thirds of the crew and 75 personnel were successfully evacuated.
The F-302 pilots landed in Caledonian territory as well, where they and the evacuees were recovered by the Caledonians and brought to a command center in the Caledonian capital. There, SG-1 devised a new plan to destroy the satellite by disabling its link to the Rand command center. After this was accomplished, using an Inertial dampener and stealth technology from an F-302 onto a Caledonian missile so as to generate an electromagnetic pulse, Mitchell and Teal'c took another F-302 to destroy the satellite.
While the operation did leave the satellite unshielded and gave Mitchell an opportunity to destroy it, Jackson was then able to negotiate a deal with the Rand leadership, whereby the Caledonians were given the planet's Stargate and the Rand agreed to dismantle the satellite. Under protest, Mitchell agreed to call off the attack after the Caledonian leader, Minister Chaska, agreed to the terms. Nonetheless, Nadal ordered an attack on Caledonia as soon as the communications link with the satellite was reestablished; but his military chief, Commander Goran Pernaux, refused, ultimately shooting Nadal dead, Pernaux having finally realized that Nadal had gone beyond being reasoned with. Though Nadal was killed as well, the deal held, ending the hostilities. (SG1: "Ethon")
The agreement lasted long enough for the Prometheus crew to depart from Tegalus, but not much longer. Soon, the Caledonians chose to launch a missile strike against the Rand, and the resulting exchange apparently buried the Stargate. The Daedalus visited the planet once more, but apparently found there was little left, suggesting that both countries had destroyed each other, resulting in the deaths of both peoples. The Daedalus presumably destroyed the satellite weapon afterwards. (SG1: "Ethon")