The Hakolians needed power sources for their technology, which was the reason behind all their invasions. They needed primitive and superstitious races for this, which they found in the humans of the 17th century. The first stage of an invasion was sending a probe (such as the Malus) which searched the surroundings and tried to stir up some chaos. (TV: The Awakening)
The next stage was sending a war creature (such as Jerak) to create an army of loyal and violent soldiers. The final stage was sending the actual Hakolians. The flaw in this invasion was that the process of creating violent individuals also caused sterility, meaning the conquered race died out quickly.