The historian Polybius (ca. 200 – 118 BC) was of Greek heritage. He was held hostage in Rome and befriended Scipio Aemilianus, whose campaigns, including the destruction of Ca rthage, he later attended. His overall theme is how and why the Romans spread their power as they did. His history describes the years 164 – 146 BC of the rise of Rome.
Polybius: Of everything that has an effect on war, the spirit of the combatants exercise the greatest influence in regard to both defeat and victory.
Source: Polybius VI, Loeb Classical Library, 2012