Read the excerpt from "A Quilt of a Country."

Once these disparate parts were held together by a common enemy, by the fault lines of world wars and the electrified fence of communism. With the end of the cold war there was the creeping concern that without a focus for hatred and distrust, a sense of national identity would evaporate, that the left side of the hyphen—African-American, Mexican-American, Irish-American—would overwhelm the right.

What is the denotative meaning of fault lines in this excerpt?

lines that are drawn when placing blame
areas of defense in times of war
long cracks in the surface of the earth
breaks in modern American society