The assault on Thessia did not go as smoothly as the Reapers' strikes against other races. While other species met the Reapers head-on, the asari resorted to dangerous hit-and-run tactics to harass their attackers. By engaging in guerilla strategies--blasting a Reaper ship, then jumping to FTL where they could not be tracked--the asari forced the Reapers to remain on the defensive.
Unfortunately, the Reapers' greater numbers allowed them to accept certain losses, so they soon ignored the attacks against them and began orbital bombardment of Thessia. This in turn forced the asari to defend their homeworld with a more traditional stance, facing the Reaper forces directly. As soon as the Reapers landed on Thessia, the harvesting began.
A swift and brutal slaughter of the asari ground forces followed. Resistance from trained biotics barely slowed the attackers down. In the end, Thessia's minimal military forces, combined with unpreparedness in the face of an overwhelming enemy, resulted in the fall of the planet.