While the expansionist philosophy's importance to both Civ and America's character shouldn't be underestimated, its not the only theme represented in the series.
For perhaps Civ's biggest contribution to gaming history, and biggest innovation was..... innovation.
Resembling a family tree of science, the Tech Tree concept helped to presented a world of dynamic change, driven by scientific progress and above all player choice.
Although the Tech tree concept existed before its creator Sid Meier adopted it there's a strong argument that Civ was responsible for popularising the mechanic. Years later after the series had become hugely successful Meier would famously say that any game should be
'A series of meaningful choices'
A quote which perfectly sums up the Tech Tree's appeal. You were constantly moving forward dramatically changing the balance of the world with your every choice.
Science is the lens through which all history is viewed in Civ. Human ingenuity channelled through science is seen as transcendent force advancing man ever forward, in the narrative of human history what once was impossible always (one day)becomes the possible. These themes of choice, the transcendental ability of science, and the power of rational thought can be traced back to the philosophies of the Enlightenment movement. These principals were the foundation of the world view of some of the most prominent of America’s founding fathers and would help shape their actions and choice in the time surrounding that great nation's birth. In particular Benjamin Franklin & Thomas Jefferson espoused it's ideas, and their influence helped weave those values into both the declaration of independence and the American constitution.
So if the players progress Tech Tree is a journey of enlightenment, where does it the end? To put it simply, the stars are your destination.