Cobra War: Book 1by Timothy Zahn
When the colony worlds Adirondack and Silvern fell to the Troft forces almost without a struggle. Outnumbered and on the defensive, Earth made a desperate decision. It would attack the aliens not from space, but on the ground—with forces the Trofts did not even suspect. Thus were created the Cobras, a guerilla force whose weapons were surgically implanted, invisible to the unsuspecting eye, yet undeniably deadly....
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