Posleen War Series by John Ringo, Michael Z. Williamson

With Earth in the path of the rapacious Posleen, the Galactic Federation offers help to the backward humans -- for a price. You can protect yourself from your enemies, but God save you from your allies!

Thanks to human valor and alien technology, the Posleen were fought to a standstill. But the invasion of Earth is just months away Only these shell-shocked survivors can save the Earth from utter devastation.

After five years of battling invaders, human civilization prepares a strike to drive the aliens from the Earth. But the Clan-Lord of the Sten has learned from the defeats humans have dealt him, and has his own plan. When he squares off against Major O'Neal, the only winner will be Satan himself.

With the defenses of the Southern Appalachians sundered, the only thing standing between the ravening Posleen hordes and the soft interior of the Cumberland Plateau are the veterans of 555th Mobile Infantry. Dropped into Rabun Pass, with a couple of million Posleen behind them and fourteen million to the front, the only question is which will run out first: power, bullets or bodies. But they have a hole card: far to the north the shattered SheVa Nine is undergoing a facelift... moreWith the defenses of the Southern Appalachians sundered, the only thing standing between the ravening Posleen hordes and the soft interior of the Cumberland Plateau are the veterans of 555th Mobile Infantry. Dropped into Rabun Pass, with a couple of million Posleen behind them and fourteen million to the front, the only question is which will run out first: power, bullets or bodies. But they have a hole card: far to the north the shattered SheVa Nine is undergoing a facelift. Rising from its smoking ashes is a new weapon of war, armed with the most advanced weaponry Terra has ever produced, capable of facing both the Posleen hordes and their redoubtable space-cruisers. Capable of dealing out hell as only SheVa Nine can. If Mike O'Neal and the other members of the 555th are going to survive, it will come down to how much Posleen butt Bun-bun can kick. Prepare to eat antimatter, Posleen-boy. less

It's a matter of trust. For thousands of years the Darhel, a warrior species conditioned to be incapable of killing, manipulated and controlled the human race. Then the humans threw off their yoke. Now, for the first time, a Darhel has been assigned to the elite Deep Reconnaissance Team commandos. Trust, in a small unit, is vital. And there was no trust to be had on either side... moreIt's a matter of trust. For thousands of years the Darhel, a warrior species conditioned to be incapable of killing, manipulated and controlled the human race. Then the humans threw off their yoke. Now, for the first time, a Darhel has been assigned to the elite Deep Reconnaissance Team commandos. Trust, in a small unit, is vital. And there was no trust to be had on either side. But when the mission encountered an alien device worth more than a king's ransom, it was the humans who betrayed the trust. Now the despised Darhel must race against the team's sniper to prevent the artifact falling into the wrong hands. The Darhel has empathic powers, superhuman strength and the speed of a cheetah. The sniper has years of experience and enormous ability. The sniper can kill. The Darhel cannot. The fate of the galaxy and the human race for the next thousand years rests on the shoulders of a Darhel. The Hero has a thousand faces, but is one of them the face of an elf? less