The God Mars Book Two: Lost Worlds - The God Mars, #2

The God Mars is a pulp-style science fiction series set in a rich world of varied cultures and colorful characters. The series is set on a partially-terraformed Mars a half-century after a man-made disaster cut thousands of colonists off from Earth. Left to survive with what few resources they had, the various groups develop different cultures at different levels of technology, ranging from the primitive to the frighteningly advanced.
And all fear the return of their home planet, because they believe Earth intentionally tried to annihilate them. In Book Two: Lost Worlds, The marooned UN survivors continue to try to establish peaceful relations with the competing factions on Mars, a challenge that becomes more urgent when a radically changed Earth finally does answer their distress calls. The governments of Earth do fear what's happened on Mars, perhaps enough to attempt once again to destroy all life on the surface, and begin moving to control the survivor groups, by military force if needed, placing the UN personnel on-planet in an untenable situation.
The situation becomes more dire when the engineer of the devastating attack that isolated Mars reveals himself, vowing to resist the return of Earth by any means necessary in order to prevent what he insists will be the end of humanity.