The period of United States history when there was much expansion is called?

Manifest Destiny

Explanation:

a term coined by journalist John O'Sullivan in 1845, was a driving force in 19th century America's western expansion—the era of U.S. territorial expansion is sometimes called the Age of Manifest Destiny.

classical republicanism, also known as civic humanism, is a form of republicanism  developed in the renaissance inspired by the governmental forms and writings of  classical  antiquity, especially such  classical  writers as aristotle, polybius, and cicero.