A Benwarian Fix

The planet Lemmus plummets into chaos, destroyed by overpopulation and pollution, forcing the Benwarians to evacuate their island nation in a search for a habitable planet . They travel for two hundred years to discover Earth. In the same condition as the planet they left, they decide to drastically reduce the human population and terraform the planet into a sustainable civilization. The United States launches an attack. Who will survive the alien onslaught?