An Oblique Approach (Belisarius, #1)

In northern India the Malwa have created an empire of unexampled evil. Guided or possessed by an intelligence from beyond time, with new weapons, old treachery, and an implacable will to power, the Malwa will sweep over the whole Earth. Only three things stand between the Malwa and their plan of eternal domination: the empire of Rome in the East, Byzantium; a crystal with vision; and a man named Belisarius, the greatest commander Earth has ever known.

From Wikipedia:
An Oblique Approach primarily revolves around Belisarius, a grand-master of the indirect approach, and a few trusted confidants forming a conspiracy to save the empire under the nose of their jealous and dangerously paranoid emperor. This is a period of seeming calm that precedes any major conflict between rivals where the conspiracy struggles to play catch up and not let its existence be discovered by friend or foe—an important deception hoping to gain both information and time. During the conflict and action (the greater part) part of this book, the Byzantines are at war with the Sassanid Persian Empire, and the general takes uncharacteristic "great risks" instead of indirection at one point to bring that conflict to a speedy end. This uncharacteristic event in turn partially exposes his conspiracy to his good friend a fellow general who becomes part of the intrigue and willing co-conspirator thereafter.