The Human Division. The Old Man's War Series

Hard-core science fiction at its very best, John Scalzi's The Human Division is the fifth in The Old Man's War series. Lieutenant Harry Wilson has an impossible mission. He must help preserve the union of humanity's colonies, in the wake of a terrible revelation. For years the Colonial Union has protected its citizens from the dangerous universe around them. But the people of Earth now know the ugly truth.
The Union deliberately kept Earth as an ignorant backwater - and as a source of recruits for its war against hostile aliens. Now, other alien races have formed a new alliance against the Union. And they've invited the incensed people of Earth to join them. Managing the Colonial Union's survival will take all the political cunning and finesse its diplomats can muster. And Harry and his team will be deployed to deal with the unexpected - for failure is unthinkable .