According to Hippolytus, Heraclitus once quipped that "War is the father of all and king of all, and some he shows as gods, others as men; some he makes slaves, others free." That's a lot to think about; however, Roman theorist, Vegetius had some suggestions in his work, The Epitome of Military Science, that were less philosophical but, perhaps, more practical.
Apparently, not much is known about Vegetius. He is known to have written the work referenced above and another one, strangely enough, on veterinary issues.
This selection, according to Vegetius, explains why the Romans were able to conquer people who were bigger, smarter, or more numerous.
August Glen-James, editor
A small force which is highly trained in the conflicts of war is more apt to victory: a raw and untrained horde is always exposed to slaughter.
In every battle it is not numbers and untaught bravery so much as skill and training that generally produce the victory. For we see no other explanation of the conquest of the world by the Roman People than their drill-at-arms, camp-discipline and military expertise. How else could small Roman forces have availed against the hordes of Gauls? How could small stature have ventured to confront Germanic tallness? That the Spaniards surpassed our men not only in numbers but in physical strength is obvious. To Africans’ treachery and money we have always been unequal. No one has doubted that we are defeated by the arts and intelligence of the Greeks. But what succeeded against all of them was careful selection of recruits, instruction in the roles, so to speak, of war, toughening in daily exercises, prior acquaintance in field practice with all possible eventualities in war and battle, and strict punishment of cowardice. Scientific knowledge of warfare nurtures courage in battles No one is afraid to do what he is confident of having learned well. A small force which is highly trained in the conflicts of war is more apt to victory: a raw and untrained horde is always exposed to slaughter.