First Contact details many events leading to the begining of the Human-Covenant War.
On the United Earth Government colony of Harvest, there is only peace and prosperity. Watched by 2 Artificial intelligences, Sif and Mack, the human colony produces much of Humanity's food. This peace is shattered when a Covenant missionary vessel, Minor Transgression, discovers a malfunctioning unmanned human cargo ship. Coordinates registered in the cargo ship's navigation computer lead the alien ship to Harvest. They registered thousands of Forerunner artifacts, including an Oracle (Forerunner A.I.). Instead of reporting this find to the Covenant Hierarchs, they intended to steal some for themselves and sell. The Kig-Yar incharge of the vessel then started raiding Human freighters. The Office of Naval Instelligence put together a trap for the pirates thinking it was Insurrectionists, leading to the destruction of the Minor Transgression.
After Sergeant Johnson came into contact the Kig-Yar pirates, they begin to prepare the Militia for a planetary invasion. Before word can be sent to the UNSC of the inpending threat, the Rapid Conversion arrives in system and begins dropping troops. Previous to the Ship's arrival, a UNSC troop transport arrives in system shortly and drops off several Pelicans fresh from Operation: TREBUCHET..