They don’t need to be fantastically superior to be virtually unkillable and dominate the skies and land. A squadron of F-22s would have been more than a match for every aircraft in the world in 1918. They would probably have completely dominated the air war over europe in ww2 as well. A single Seawolf would annihilate every single ww2 fleet so long as it was resupplied torpedoes regularly.
A supply train that will get very short as soon as they set up the manufacturing facilities they brought with them on whatever portion of the earths landmass they control first(most of the southern hemisphere, most likely, since few countries could put up much of a fight). If they are coming here, they would be prepared to live here, which means building the tools they need, since I doubt this would be a back to the farm movement.
It is also a certainty that even if they didn’t include certain weapons in with the ships prebuilt, there would be full plans on how to build whatever they needed, and whatever machinery they had would be up to the job. They did, of course, bring the machinery… You expect them to come to a hostile world and subjugate it and/or exploit its natural resources without having capacity to build new equipment?
Actually sending combat vehicles along on the ships is just wasted mass, making the ships too big. They would have to have mining and manufacturing capability anyway, since they couldnt rely on the natives to build anything more than their dinnerware. Send factory ships instead of combat vehicles, land on on of the outer moons for a year or two, or ten, while you monitor electronic traffic to gain intel, and then procede.
Also remember that the aliens control the earths orbit. Anything on the surface larger than a man would be a target. All of our manufacturing capacity would be gone in days. All of our satellites would be gone within days. All undersea cables, gone. All frequencies we use, jammed, or monitored by computers far, far superior to our own. There would be no resupply for any of our units. No command and control beyond dispatch on horseback, but good luck finding horses. Of course, gas is going to be gone within weeks, since they would send precision strikes down to cut pipelines, refineries, depots, tanker ships, etc, so food would stop moving along with all military vehicles. Our navies would be gone, since ships are incredibly easy to spot top down.
No. Earth would be screwed against any planned invasion, even if they only expected 15th century man. Turtledoves books only worked because he envisioned aliens with basically year 2000 technology, plus a couple added features such as fusion engines.
