Piano players who do drills when they are children to improve dexterity in their left hands experience as a result.

I believe the answer is: brain changes that greatly improve dexterity

As we are conditioning ourselves to do something with a certain part of our body, our brain would eventually improve and adjust itself to make us able to do that thing better, even on automatic function. The same thing happen to soccer youths who are continuously trained to use both of their leg to make a maneuver ever since they're little.

first, the romans stayed isolated because they had to defend their own territories from neighboring cities that were harassing them, like the etruscans. second, once the romans managed to defeat their neighbors, and started to expand their territory, they started to build roads and bridges. finally, now that the romans have covered their territory with roads functioning as highways, they could mobilize their legions faster to assemble larger military units in a specific place.