Military is the best option because of a common recurring theme: conquest. The Romans were known for their legions, expanding to take over the entire Mediterranean sea coastline and more beyond that.
The Aztec Empire was born out of a defense compact between three ancient Mesoamerican cities, and rapidly expanded to fill most of Central and Southern Mexico.
The Incan empire, which they called the "Land of Four Corners," was based on conquests which expanded to take control of most of the Andes Mountains.
