Orionblamblam said:
Of course it's hard to say exactly how such a battle would go in real life. Maybe the aliens will have force fields like those in "Independence Day," and our weapons will be worthless. Or maybe they won't, and their vehicles will be made out of materials we can comprehend. In which case, a 30mm armor piercing round out of an A-10 will cut through it like a 30 caliber round out of a WWI-era biplane cutting through a 2010 Gulfstream.
All of these alien invasion stories are just copies of HG Wells’s original: aliens arrive from nowhere, attack everyone, take over and are brought down by some small, unexpected irritant. The only innovator was V that added the Nazi like takeover of society in place of general attack. Since these stories are being written without a concern for the suspension of disbelief of the small slice of the potential audience with a medium or high level of military and scientific knowledge they can be well within the many shades of bullshit and still a success.
From our own human history there aren’t really any examples of first contact being invasion. Animals will invade straight off but not humans. Usually we require some more benign contact before competition for resources leads to conflict. One would assume that any interstellar/interplanetary travel capable species would be just as curious as us so would want to communicate and share before economic pressures lead to armed conflict.
As to military disparity even space aliens need to follow the laws of physics. They may have far more capable weapon systems but release enough heat through chemical or atomic reaction and it will destroy what they have built. For space travel the important technological requirements are thrust and life support. They may not even need the later if they have evolved a hibernation capability. Neither of these requirements naturally suggest highly capable ISR, force protection and lethality.
They may even be inferior in military state of art. In one of his short stories Asimov drew the military comparison between waring Greek city states and the Persian Empire and disunited earthly nations and some space alien empire. A world crossing space alien empire may not be very belligerent because of political unity and a lack of immediate threats.
The most valuable things on earth are biological life and human society. For any invader these interrelated systems are far more important than inanimate natural resources which are freely available in large quantities in space. Human society produces enormous amounts of surplus chemicals, metals, proteins and technological items (etc). Any rational invasion would seek to capture life and society rather than simply eradicate it.
So any alien invader would face the same complexities the west has in trying to civilise Iraq/Afghanistan. There is no Conquistador ‘capture the god-king’ manoeuvre you can pull on earth as a whole. You would have to go street by street and island by island. Fighting humanity would not be a very pleasant experience because our military potential is so widely proliferated.