We, the Hrimfaxi, an extensive interstellar empire, are invading the Earth!
However, these pesky humans have proved themselves to be extremely good at pouring out dakka and making things go kaboom. Despite the technological advantages the we had during the first days of the invasion, they seemed to be adapting and dragging the ground war on and on.
So why shouldn't I, the commander of the invasion just claim Earth a lost cause and declare Exterminatus? After all, Earth is just one small planet in the entire galaxy.
My space forces consists of a rather small flotilla of space ships armed with weapons on the equivalent of Earth's strategic nuclear weapons, enough to make Reagan and Gorbachev blush. I have a small shipyard in the solar system so I can procure more as I needed.
My soldiers, on the other hand, are cloned, mindless drones, controlled by me through a psychic chain of command. Commanders like me have free will, while sub-commanders further down the chain have decreasing levels of free will. They are organized fairly conventionally: infantry, armor, artillery, air force, etc. I have cloning tanks at my disposal to replenish losses.
Standard expansion of Hrimfaxi sphere of influence and adding humans to the collective psychic network of Hrimfaxi systems. Secondary objectives include cataloging the genetic properties of life on earth, with is largely complete through abductions. The reasons are so buried underneath bureaucracy, ancient history, and indoctrination that most Hrimfaxi don't even know why they are invading other worlds other than they must or their superiors will have them executed. In fact, it is highly debatable if the higher spheres of the bureaucracy psychic network can be even fathomed under the mindset of an individual being.