Michael G. Thomas - Siege of Titan (Star Crusades, Book 1)

Siege of Titan is the first book in the epic five part science fiction series that chronicles humanity's first interstellar confederation that is torn apart by religious strife and extremism.
In the first book of the Star Crusades series, the bloody struggle is told through the eyes of an illegal pit fighter named Spartan, who is forced to choose between incarceration or military service. Choosing the Confederate Marine Corps over servitude he begins his thirty long weeks of training. Together with his group of marines, they are honed into a force ready to be thrown into the meat grinder on the newest and greatest planet in the Confederacy, Proxima Prime. His term of service will soon become one of violence and carnage, as the entire sector is ripped apart by a brutal and virulent religious movement and its shocking plans.
What starts as a series of suicide bombings and hijackings through the Confederacy quickly turns into a full-scale revolution on Prime that threatens the entire planet and quickly spreads to its orbital Naval Station and beyond. As every warship and military transport in the area is rushed to its defence, the fate of the planet and the entire sector lie in the hands of a small number of rookie marines.
Siege of Titan is a classic work of realistic military science fiction set against the background of the first interstellar human empire.