In my setting, a near-human species attempts to invade modern-day Earth. With a few, very strange, outlier exceptions they are limited to WWII-era technology, and cultural/societal restrictions keep them from adapting and improving beyond this. The exceptions are the absolute minimum necessities that allow for large-scale space travel, and planetary take off/landing. These processes are generally crude in nature, but effective enough to get the job done.
They have the ability to land their WWII era aquatic ships in the oceans in a controlled descent. They have the ability to send in aircraft in drop pods that deploy once reaching a proper altitude. Similar technology allows paratroopers to be deployed. Other land forces must be deployed from a water ship. Communication with the orbital fleet is limited to communication aquatic ships and communication towers that can be established planetside. Radio and encryption technology are otherwise roughly on par with WWII.
They have WWII-style artillery that can fire from space. Similar to America during WWII, they have used computers to calculate required trajectories for angle, elevation, etc, and fire the shells according to lookup tables and books. The largest space artillery pieces equaling that of the Schwerer Gustav. The shells are specifically designed to withstand atmospheric entry.
Edit: Per discussion with Mołot, the accuracy could be no more than a 20 mile radius. The time from firing to impact would be roughly half a day, allowing developed nations plenty of time to respond. Anti-ballistic missiles would be a natural defense against the artillery, against which the only real counter (that I can think of) is to simply exhaust the anti-ballistic supply with enough artillery shells.
On principle of honor, they refuse to perform acts of mass destruction or non-traditional warfare, such as gas attacks, atomic warfare, or accelerating large objects into the surface of the Earth.
Potentially I can make the invading force number millions, billions, or trillions in size. Similarly, the ammount of non-perishable supplies they bring with them can upscale as needed.
The goal of the invading force is to subdue the armies of Earth and occupy the planet, not mass genocide.
Ignoring the logistical issues with commanding and mobilizing forces on such massive scales with such antiquated technology, how much larger must the invading force be to be more or less on equal footing against Earth? What would be the primary challenges this invading force would face?