But how can you be sure?
It is possible that most JLA/U members are, in fact, pre-Cylons (or post-Cylons?!), hiding amidst the Earth's population to enact their evil conquest plans. It would explain their toughness. And, of course, there's also the chance that the Cylons are working with Brainiac. Artifical Intelligences of a feather flock together, after all.
As for Shivans, they are clearly responsible for the destruction of Krypton, due to their (Kryptonians) subspace experimentation (Phantom Zone).
AAAAAHHH IT IS ALL CONNECTED AAAAAH
Maybe Atlantis is the first traction city.