Res Gestae

But in this state of indecision he was kept back by many delays, since the Persian king was waiting on the other side of the Tigris until the signs from heaven should warrant a move; for if Sapor had crossed the river and found no one to oppose him, he could easily have penetrated to the Euphrates; besides this, since he was keeping his soldiers in condition for civil war, he feared to expose them to the dangers of an attack upon a walled city, knowing by experience the strength of its fortifications and the energy of its defenders.