Sigma Octanus IV
From Halopedia, the Halo wiki
Sigma Octanus IV, located in the Sigma Octanus star system is an out-of-the-way colony planet. With one moon and a population of nearly thirteen million UNSC citizens. It features warm, humid atmosphere that is perfect for growing warm weather crops, like rice. The planet has many rice fields. Much of the planet is a dense jungle, and it appears that a meteor containing Forerunner artifacts crashed on the planet before the Human's arrival.
It came under attack in July of 2552. One of the planet's major population centers, Côte d'Azur, was attacked by Covenant ground forces, and the Spartans were deployed to find out why the Covenant were so interested in the city. It turns out that the museum of the city held a Forerunner artifact that the Covenant wanted and the Spartans were able to use this distraction to level a Covenant Crusier with a HAVOK Nuclear Warhead. Later on, it is revealed that the Forerunner artifact contained directions to Installation 04.
To this day, it is the only human-controlled world, with the exception of Earth, that has survived a Covenant assault. However, one must note that the Covenant were not attempting to simply destroy them. The assault on Sigma Octanus was a distraction to remove the Forerunner artifact, and it seems likely that the Prophet of Regret was looking for Forerunner installations (The Ark most likely) on Earth, rather than bringing an invasion fleet.
For complete information on the attack, see The Battle of Sigma Octanus IV.
