This is book number 1 in the Cobra War series.
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. And the Moreau family were the most famous of the Cobra warriors.
Long after victory over the Troft was achieved, the Cobras made common cause against their former adversaries against a new enemy. Their reward was three planets that would be a home for the Cobras, whose deadly powers made them too dangerous to feel at home on Earth. Now, years had passed and not everyone on the Cobra worlds thought that the Cobras were worth the high cost of providing their training and maintaining their existing built-in weaponry, let alone supporting research to improve the Cobra weapons, and possibly even put an end to the negative effects of that built-in weaponry, which caused Cobras to die much too young. Many who had never known interplanetary war were convinced that the Cobras were not needed at all. That was a grave miscalculation, because a new menace was approaching, one that even the formidable Cobra warriors might not be able to defeat.