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who wrote the primary source document

In the history of ideas or intellectual history, the main primary sources are books, essays, and letters written by intellectuals; these intellectuals may include historians, whose books and essays are therefore considered primary sources for the intellectual historian, though they are secondary sources in their own topical fields.

Explanation:

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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.