The Human Division (Old Man's War #5) by John Scalzi

Following the events of The Last Colony, John Scalzi tells the story of the fight to maintain the unity of the human race.
The people of Earth now know that the human Colonial Union has kept them ignorant of the dangerous universe around them. For generations the CU had defended humanity against hostile aliens, deliberately keeping Earth an ignorant backwater and a source of military recruits. Now the CU’s secrets are known to all.

Other alien races have come on the scene and formed a new alliance—an alliance against the Colonial Union. And they’ve invited the people of Earth to join them. For a shaken and betrayed Earth, the choice isn't obvious or easy.