The Sentinels: Book One: The Human Agenda - The Sentinels, #1

Humans have always thought of themselves as the dominant species on their planet and even maybe in their solar system. Close to civil war and on the brink of a third world war, an alien species visits Earth and gives all humans an ultimatum. Join an extensive collection of alien races or face destruction by one of the fiercest warriors in the known universe. The decision is up to every human worldwide, join or be killed.
With such a decision on their hands, what will the outcome be? What consequences could come of either outcome?