The planet had been originally under human control with the orks landing on the planet over two hundred years ago. A swift action by the Iron Snakes chapter ended the ork invasion with the orks retreating into the mountains. This time though the Tau had taken a special interest in the planet for its farmland and minerals. So the Tau had decided to invade the planet. The Tau decided to land in the mountain range, because the mountain range would conceal the Tau movements and be able to strike anywhere. Instead the growing number of orks wiped out the Tau expedition force. With the all the ork clans fighting under one banner they poured out of the mountains to the unsuspecting humans below. Within four days the ork hordes had wiped out 30 of the planets population and 60 of the PDF.
An emergency message had been sent immediately after the first ork was sited. Three space marine chapters responded as well as the Cadian 1st and the Steel Legion 4th regiment. The three marine chapters were the Blood Angels, Ultramarines and the newly formed chapter Blood Fists. Not only did the humans receive reinforcements but also the Tau had lost their expedition force and planned to send a bigger fleet to capture the planet. Fate would have it that both the human and the Tau fleets would arrive in the Cidona system at within a day of each other.
Don't worry this is just the history. There will be more action in coming chapters if I get some feedback. The next chapter will have the IG and the Tau fleets engaging before sending their armies down to the surface.