Their early interstellar history is one of mistakes and failures. Eventually they matured out of that stage into a race of interplanetary conquerors who desired to take down the Galactic Council.
Reconnaissance Period on Earth
Their attack on Earth was spearheaded by a "fifth column" of spies and impersonators who weakened Earth both by framing people, and by replacing government officials, and using their power to slip in troops, and focus negative attention on people that were considered a potential threat.
After the Confessor revealed the ruse, they were eventually forced to retreat.