Virtus in the Roman world was often cited, by the Romans themselves, to be their defining attribute that allowed them to conquer the Mediterranean. Virtus’ meaning changed throughout the Roman Republic as different successful methodologies came into usage, and eventually the word virtus focused solely on those who were successful, rather than their own moral or practical character.
Schrader, Kyle W.
"Virtus in the Roman World: Generality, Specificity, and Fluidity,"
The Gettysburg Historical Journal: Vol. 15
, Article 6.
Available at: http://cupola.gettysburg.edu/ghj/vol15/iss1/6