Call to Arms is the sixth book in the epic 'Nexus' series, part of the growing Star Crusades universe that chronicles humanity's first steps away from Alpha Centauri and beyond.

The Great Biomech War is now inevitable and the citizens of the Alliance ready themselves for what is to come. Ships and soldiers from a hundred worlds set their eyes on Helios, some looking to defend it and others making their plans to bring about its destruction. As each day passes, the defenders around Helios grow, and each day brings the prophetic comet closer to them. The remaining Biomechs have their own plans, and worlds as far away as Earth now feel their wrath.

Spartan arrives on Earth to find a place as backward and corrupt as any he has seen before. The colonies of the Earthsec territories operate with an antiquated military and a great distrust of the wider Alliance that makes them easy prey to the Biomechs. As he recuperates from his tortured ordeal a final message arrives from Mars. Enemy ships have smashed through the Rift and in hours all contact is lost with the Martian colonies.

Meanwhile, on Prometheus the elite forces of the Navy and Marine Corps, under the command of Colonel Teresa Morato and Admiral Churchill, hatch a plan as daring as anything seen in the days of the Great Uprising. A plan that has been weeks in the making and could see the Alliance move to a position of unequalled strength in readiness for the battle to come.

Skirmishes and battles spread throughout the Alliance, but it is what lies beneath the surface of Mars that will change the fortunes of the war in ways that could have never been imagined.