Robotech: The Macross Saga: Doomsday (Robotech #4-6) by Jack McKinney

The electrifying second Robotech omnibus. Experience the struggles and triumphs of the Robotech Defense Force, led by the brilliant but unorthodox Captain Gloval, as they fight against the alien Zentraedi—only to find themselves fighting alongside their former enemy in a struggle against total annihilation.

BATTLEHYMN
For two years, Captain Gloval and his crew had been chased through the solar system, only to be made to feel unwanted and maligned when they returned to Earth. Now, in direct violation of Council orders, Gloval commands the Super Dimensional Fortress to rise from its landing site…

FORCE OF ARMS
Supreme Commander Dolza had amassed the largest fleet of Zentraedi warships the Universe has ever seen… and all their weapons are aimed at Earth!

DOOMSDAY
A war without victors. A war that brought two races to the brink of extinction. A war without spoils, save for the devastated Earth itself... A new-age ark, the Super Dimensional Fortress, returns to its ravaged homeworld. Those who lived through Armageddon began the painstaking process of reconstruction. But the humans had the Zentraedi to help them, former enemies who shared a common goal—survival!

But all was not well in this bravest of worlds... Unaccustomed to a life without warfare, many of the alien giants revert to their old ways. One alien in particular promises to lead the Zentraedi back to their former glory —Khyron. A hero reborn vows to pick up where Dolza had left off!