The Xindi are actually comprised of 5 sub-species: Primates, arboreals, aquatics, insectoids and reptilians. There was once a sixth species, the Avians, but they went extinct when the Xindi homeworld was destroyed in a civil war.
After being told by the Sphere Builders that Earth would destroy the Xindi in 400 years, the Xindi decided to launch a pre-emptive strike. They test-fired a powerful weapon on Earth, killing 7 million people. Fortunatly, a normally antagonistic temporal agent told Captain Archer the location of the Xindi; a mysterious region of space known as the Delphic Expanse.
Archer and Co. sent out to stop the Xindi from launching their planet-incinerating superweapon against Earth. After months of searching, skirmishes and other setbacks, Archer was able to convince the greater number of the Xindi species they had been manipulated into attacking Earth by the Sphere Builders.
The Sphere Builders were on the verge of taking over the galaxy in the 26th century, but were defeated by the United Federation of Planets (of which Earth was the founding member). But the Sphere Builders saw this future, as they had technology that allowed them to forsee alternate timelines. So they tried to have the Xindi destroy Earth to change history in their favour.
The Xindi-Reptilians still attempte...