Complex sentence: The series is set in the near future, where a second civil war has turned the island of Manhattan into a demilitarized zone, caught between forces of the United States of America and secessionist "Free States" armies.
Split into simpler sentences: The series is set in New York City, sometime in the near future and in the midst of a civil war that has turned the island of Manhattan into a demilitarized zone. The conflict concerns two primary forces: the federal government of the United States of America and the "Free States" armies.