None but Man by Gordon R. Dickson 1977
Border warfare on the Pleiades:
The Pleiades Planets, the last outpost of the human frontier, has become an interstellar hotbed.
The unhuman Moldaug Ambassador to the Tri Worlds Council warned that the Pleiades belonged to the Aliens, and that the Moldaug are ready to attack the whole human race if the Frontier is not evacuated immediately.
Would the diplomats of the Old Worlds sacrifice the border colonies in a panic attempt to avert galaxial war? If so, one Frontiersman was prepared to wage a guerilla war all his own!