In my story, humanity comes under attack by extraterrestrial forces determined to sieze Earth's resources (water particularly). In my research it seems to be more plausible that aliens would attempt to exterminate humans by introducing a killer virus or Weapon of Mass Destruction rather than attempt a full-scale invasion.
I do not want to use the quite old concept of an alien killer virus or some giant space laser since contaminating or destroying the earth's surface is not their objective. In a previous question, i wanted to know if aliens would use self-replicating drones to exterminate the human race.
My next question is, if aliens did use these drones to invade, would they immediately target and eliminate humans or would they attempt to take out our infrastructure?