Bailey Cove Sci-Fi Book Club: Probability Moon

Earth is an environmental disaster area when humanity gains new hope: a star gate is discovered in the solar system, built by a long-gone alien race. Earth establishes extrasolar colonies and discovers alien races--including the warlike Fallers, the only spacefaring race besides humans. Mysterious, uncommunicative, and relentlessly bent on humanity's extinction, the Fallers have mastered the star gates, and are closing in on earth.