Doom Series by Dafydd ab Hugh, Brad Linaweaver

The Gates were there on Phobos when mankind first arrived. Inert, unyielding, impossibly alien constructs, for twenty years they sat lifeless, mute testaments to their long-vanished creators, their secrets hidden. Then one day, they sprang to life... Meet Corporal Flynn Taggart, United States Marine Corps; serial number 888-23-9912. He's the best warrior the twenty-first century has to offer, which is a damn good thing... moreThe Gates were there on Phobos when mankind first arrived. Inert, unyielding, impossibly alien constructs, for twenty years they sat lifeless, mute testaments to their long-vanished creators, their secrets hidden. Then one day, they sprang to life... Meet Corporal Flynn Taggart, United States Marine Corps; serial number 888-23-9912. He's the best warrior the twenty-first century has to offer, which is a damn good thing. Because Flynn Taggart is all that's standing between the hell that just dropped in on Mars and an unsuspecting planet Earth... less

They were creatures seemingly spawned straight from the pits of Hell - demons, zombies, fire-breathing imps - all too horrifically close to the stuff of nightmare to be real. But they were. And on the inhospitable moons of Mars, Corporal Flynn "Fly" Taggart, Earth's last line of defense against a seemingly inexhaustible supply of alien warriors, beat them back almost single-handedly.But Taggart discovers that the war had barely begun.. moreThey were creatures seemingly spawned straight from the pits of Hell - demons, zombies, fire-breathing imps - all too horrifically close to the stuff of nightmare to be real. But they were. And on the inhospitable moons of Mars, Corporal Flynn "Fly" Taggart, Earth's last line of defense against a seemingly inexhaustible supply of alien warriors, beat them back almost single-handedly.But Taggart discovers that the war had barely begun... for while he was fighting them on Mars, the hellish creatures had established a beachhead on Earth itself. Now, with the aid of a fourteen-year-old female computer genius, an unrepentant Mormon sniper, and the best soldier in this woman's army, Fly Tagart must defeat the invaders - and their treacherous human allies - yet again... less

Hawaii -- the last outpost of civilization on an Earth overrun by demons, traitors, and nightmarish creatures straight out of the pits of Hell. Humanity seems doomed to a bloody ending. Then Hawaii receives a message from aliens claiming to be on our side of the battle. Our last chance: make contact.The only man for the job -- Corporal Flynn Taggart, U.S. Marine Corps -- "Fly" to his friends. He led the fight against the demon invaders when they swarmed through the Gates at Phobos Base... moreHawaii -- the last outpost of civilization on an Earth overrun by demons, traitors, and nightmarish creatures straight out of the pits of Hell. Humanity seems doomed to a bloody ending. Then Hawaii receives a message from aliens claiming to be on our side of the battle. Our last chance: make contact.The only man for the job -- Corporal Flynn Taggart, U.S. Marine Corps -- "Fly" to his friends. He led the fight against the demon invaders when they swarmed through the Gates at Phobos Base. Now Fly has to face the toughest task of his dirty career: return to Phobos -- and fight his way past those demons to contact mankind's would-be-rescuers... less

They left behind everything that mattered to them-- friends, lovers, country-- to journey to the stars. Now Sergeant Flynn Taggart and Pfc. Arlene Sanders, USMC, have reached their destination... the homeworld of the demon invaders who destroyed Earth. But there, they find a scene of destruction that rivals any they left back on Earth. And suddenly, "Fly" and Arlene find themselves face-to-face with an even deadlier enemy than the demons they came to fight. The war for Earth is over... moreThey left behind everything that mattered to them-- friends, lovers, country-- to journey to the stars. Now Sergeant Flynn Taggart and Pfc. Arlene Sanders, USMC, have reached their destination... the homeworld of the demon invaders who destroyed Earth. But there, they find a scene of destruction that rivals any they left back on Earth. And suddenly, "Fly" and Arlene find themselves face-to-face with an even deadlier enemy than the demons they came to fight. The war for Earth is over. But the battle for the stars has just begun... less