According to the textbook, the dsm-5 has been criticized for

the anasazi indians were real pioneers and created roads and irrigation systems for watering their crops. they were also great architects and the cliff dwellings are one of their biggest achievements.

explanation:

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.