Semper Mars (Heritage, Book 1)

The Year is 2040.
The Marines have landed on Mars to guard the unearthed secrets of an ancient and dangerous alien race: Ourselves.
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 interest with lethal force if necessary.
Because great powers are willing to devastate a world in order to keep an ancient secret buried. Because something that was hidden in the Martian dust for half a million years has just been unearthed . . . something that calls into question every belief that forms the delicate foundation of civilization . . .Something inexplicably human.