Hmmmm...
I think I can build that up a bit better. For one thing, if we're doing the whole "They were the gods and monsters" thing, then that would leave huge gaps of time between appearances.
I think we can break it up into two major incursions. For one, we'd have the Deific Incursion where primitive humans mistook them all for gods. And then we would have the Monstrous Incursion which probably started the Dark Ages, and thus Humans labeled the Neuroi a little more appropriately.
The interesting thing is that the Monstrous Incursion could also resolve a lot of questions about the world building going on in Strike Witches. IIRC, the Silk Road was still a thing back in the Dark Ages, so word would have gotten around and Kingdoms A, B and C were not the only ones troubled by the Monsters - Kingdoms X, Y, and Z were suffering as well. Therefore, it would have only been natural if all these kingdoms formed alliances where knowledge and materials were exchanged and even a few marriage contracts among the nobility to better shore up these alliances.
That last bit is IMPORTANT. Not only is it one of the more likely things to happen - nothing draws strangers closer together by making them into family - but it would have had a myriad of other affects. For one thing, Royal lines that would have died out due to inbreeding wouldn't have. Also, there would not have been as much of the dangerous nationalism that set nations like France and England at each others' throats - that energy would have been funneled into the more constructive pursuit of seeing who can kill the most monsters. (And in the rare case you had someone that was so ambitious that they wanted to rule more than their fair share once they inherited their respective throne... pretty sure they'd be quietly dealt with once they were found out.)
After the Monsters finally leave, you would have the natural result of so many countries switching from a war-footing to peace... The Renaissance. Only with one important difference: nobody is really at anyone else's throat at this time. They're all still kinda buddy-buddy after that hard-fought war against the Monsters.
Centuries pass, and eventually people find reasons to get pissed off at each other. Liberia (USA) is founded, but a lot of the old empires remain out of habit (Britania, Gallia, Karlsland, Romagna, Orussia). And then the monsters came back - this time, having a lot more development, we understand them as aliens from another world and call them Neuroi.
So, as for the Neuroi themselves and their reasons? Well, I had the thought that they could be after Earth for those oceans filled with Deuterium, just waiting to be filtered out. Sure, you can get it out of a gas giant, but that has hazards associated with it that makes using a terrestrial water world like Earth much more attractive. It has radiation shielding for one, and that thick atmosphere protects it from all the smaller space rocks, so you'd only have to worry about the really big ones. Also, it's loaded with iron, silica, and other useful materials for a silicon-based species, making the planet into a one-stop shop for everything they'd want.
There's just one problem - it seems to be infested with a rather virulent pest that likes to attack anything that is alien and unknown. They tried once and were either beaten back or the pests wouldn't stop worshiping them. So they left in hopes that something might happen to them - like a mass die-off, or some new species that was easier to deal with forces the pests out of the picture. Unfortunately, not only were the pests still there when they came back a thousand years later, they had somehow become stronger. The pests were not only growing rapidly, but they were making tools and infrastructure. Naturally, there was a panic as the Neuroi tried to destroy the empires of the pests. They succeeded, but only on the surface. Instead, the pests went to ground, founded new empires, and kept on fighting back, getting smarter and more capable with each battle. Every time the Neuroi stepped up their game, the pests rose to meet the challenge.
Dejected, the Neuroi abandoned the project and settled for the long game of using the gas giants and the asteroid belt.
Until one day several centuries later, a hive went rogue and went to Earth, determined to see if it can co-exist on some level with the pests. For sure, it may be an antagonistic relationship, but maybe it was possible they could communicate - the pests did, after all, just recently start using radio waves for communication.
Thoughts?
I think I can build that up a bit better. For one thing, if we're doing the whole "They were the gods and monsters" thing, then that would leave huge gaps of time between appearances.
I think we can break it up into two major incursions. For one, we'd have the Deific Incursion where primitive humans mistook them all for gods. And then we would have the Monstrous Incursion which probably started the Dark Ages, and thus Humans labeled the Neuroi a little more appropriately.
The interesting thing is that the Monstrous Incursion could also resolve a lot of questions about the world building going on in Strike Witches. IIRC, the Silk Road was still a thing back in the Dark Ages, so word would have gotten around and Kingdoms A, B and C were not the only ones troubled by the Monsters - Kingdoms X, Y, and Z were suffering as well. Therefore, it would have only been natural if all these kingdoms formed alliances where knowledge and materials were exchanged and even a few marriage contracts among the nobility to better shore up these alliances.
That last bit is IMPORTANT. Not only is it one of the more likely things to happen - nothing draws strangers closer together by making them into family - but it would have had a myriad of other affects. For one thing, Royal lines that would have died out due to inbreeding wouldn't have. Also, there would not have been as much of the dangerous nationalism that set nations like France and England at each others' throats - that energy would have been funneled into the more constructive pursuit of seeing who can kill the most monsters. (And in the rare case you had someone that was so ambitious that they wanted to rule more than their fair share once they inherited their respective throne... pretty sure they'd be quietly dealt with once they were found out.)
After the Monsters finally leave, you would have the natural result of so many countries switching from a war-footing to peace... The Renaissance. Only with one important difference: nobody is really at anyone else's throat at this time. They're all still kinda buddy-buddy after that hard-fought war against the Monsters.
Centuries pass, and eventually people find reasons to get pissed off at each other. Liberia (USA) is founded, but a lot of the old empires remain out of habit (Britania, Gallia, Karlsland, Romagna, Orussia). And then the monsters came back - this time, having a lot more development, we understand them as aliens from another world and call them Neuroi.
So, as for the Neuroi themselves and their reasons? Well, I had the thought that they could be after Earth for those oceans filled with Deuterium, just waiting to be filtered out. Sure, you can get it out of a gas giant, but that has hazards associated with it that makes using a terrestrial water world like Earth much more attractive. It has radiation shielding for one, and that thick atmosphere protects it from all the smaller space rocks, so you'd only have to worry about the really big ones. Also, it's loaded with iron, silica, and other useful materials for a silicon-based species, making the planet into a one-stop shop for everything they'd want.
There's just one problem - it seems to be infested with a rather virulent pest that likes to attack anything that is alien and unknown. They tried once and were either beaten back or the pests wouldn't stop worshiping them. So they left in hopes that something might happen to them - like a mass die-off, or some new species that was easier to deal with forces the pests out of the picture. Unfortunately, not only were the pests still there when they came back a thousand years later, they had somehow become stronger. The pests were not only growing rapidly, but they were making tools and infrastructure. Naturally, there was a panic as the Neuroi tried to destroy the empires of the pests. They succeeded, but only on the surface. Instead, the pests went to ground, founded new empires, and kept on fighting back, getting smarter and more capable with each battle. Every time the Neuroi stepped up their game, the pests rose to meet the challenge.
Dejected, the Neuroi abandoned the project and settled for the long game of using the gas giants and the asteroid belt.
Until one day several centuries later, a hive went rogue and went to Earth, determined to see if it can co-exist on some level with the pests. For sure, it may be an antagonistic relationship, but maybe it was possible they could communicate - the pests did, after all, just recently start using radio waves for communication.
Thoughts?