The Great Crusade finds Earth. What do you do?
So let's say the Great Crusade finds Earth. We're not the Earth of the lore etc, let's just say for arguments sake, that it's a very strange co-incidence that we have an identical solar system and history etc. Just ignore that and move on! Cool.
So they've turned up, little to no Legion support is present at the time, just the Imperial Army, but let's say led by someone with a hefty amount of pull, someone that could quite easily request Legion support and it would come.
Knowing what you know about 30k/40k, what will happen (or might not happen potentially) during the events of the Horus Heresy etc. What would you do? Would you seek out the Imperial representatives and tell them all you know, likely be branded as crazy or delusional. Or would you hold your tongue? Generally this is going to depend on other factors as well, such as how do you think the nations of Earth would react? If the big nations defy the Imperium and defend themselves, not wanting to be 'enlightened', if so, going to be hard contacting the Imperium, would you rebel against them, or defect from your nation and join the Imperium as a collaborator.
Bear in mind, if the Earth does go to war, and Legion support is called, you've got no idea at all who will turn up. Or if you convince someone enough to get a Legion response to your claims, you may get a Ultramarine, who may well listen to your theoreticals, or you might get a Night Lord who will simply flay you alive.
Go. Let's try and get some discussion back into this board.