The human ships were outnumbered 500 to 150 (they usually won an engagement if they were outnumbering them 3 to 1) , the only thing they had going for them were the Super MAC cannons in orbit. A few ships got through to the atmosphere and dropped about a million covenant on the ground. Now the Spartans always won ground engagements, but having to protect the nuclear reactors (powering the MAC), they eventually had to move and leave the reactors to be destroyed (they set off a nuke themselves). This made the super MAC guns useless and got the rest of the fleet wiped out. Then the covenant did what they always do. Glass the planet.