Implacable (The Lost Fleet-Outlands 3)

As far from explored space as any human has ever been, Geary and the Alliance fleet are on their own, protecting a diplomatic mission in territory belonging to an alien species with still-unknown motives.
His already complex and dangerous mission is further imperiled by deadly challenges from other human factions seeking to harm or exploit the aliens. When another alien species whose technology is far more advanced than humanity’s arrives, the stakes are raised to the highest possible level.
Yet the most serious trouble comes from an unexpected source.
Presented with orders to carry out actions he believes not only are mistaken but would be contrary to the ideals of the Alliance, Geary has to decide whether he must invoke the power that his long-revered name holds—even though this might endanger his entire fleet, tear apart the Alliance, and destroy everything he has fought for.