Scientists have discovered something astonishing in the subterranean ruins of a sprawling Martian city: startling evidence of an alternative history that threatens to split humanity into opposing factions and plunge the Earth into chaos and war. The USMC - a branch of a military considered, until just recently, to be obsolete - has dispatched the Marine Mars Expeditionary Force, a thirty-man weapons platoon, to the Red Planet to protect American civilians and interests with lethal force if necessary.