The funny thing about Earth, at least in the view of the multi, is that when it gets in trouble, it tends to get into it sideways.
Which is strange, once one who knows thinks about it. It is not that Earth is the first planet to get invaded. Far from it. Nor necessarily, on the grand scheme of thing, is it the most important. It is, however, the one most... numerous and varied, so to speak, on the alt-directional ways when it happens.
It is quite fascinating to behold, at least in comparison to the number of times it isn't threatened. It's almost like mankind's martial spirit's desperation for a worthy opponent in light of it's maturing civility is drawing attention of those willing to answer the challenge broadcasted across space time.
Over here, for instance, has an Earth that has been plagued by alien invasion for millennia, yet it's outward development is unchanged from the base because of an incessant need to keep knowledge of the danger quiet in the name of "security", though the examples where knowledge had gotten out do bare evident to the wiseness of this move.
Over there, in contrast, are multiple little Earths where the planet's previous untouched nature is rudely despoiled by less scrupulous visitors. Sometimes it is a subtle sparring of the minds, the dance of cloak and dagger. Others are more roar and fire of military muscle (usually roar for the visitors, and fire for the poor Earthlings, though exceptions do exist), often with bodycounts in the high percentages.
Each time for those Earths, though, the destinies diverge greatly from one another in the aftermath, with humanity either readying for revenge, desperately prepping for the next wave, stoically clawing away from under an alien boot (and it's usually a boot. The ones with claws tend to have other plans), or just meekly picking up the pieces. And that assumes humanity survives.
And it's not just the nature of the invasion that varies so much, it's the nature of the aliens invading, and how humanity defends itself.
Looking back at the Earth plagued by invasion as an example, it seems as though if it is extra-terrestrial, and it is sentient, then it wants the little blue planet innocently circling around its yellow star three orbits out. The reasons range from "saving it from itself" to "it's not mine, so I want it" to "it used to be mine, I want it back" to "pineapple square to HG sewing mishkeainlo blue" to "1011100010010110".
And in the face of this constant varied intrusion, humankind has tried varied secret responses, the only common factor between them being that they are successful before falling apart spectacularly, leaving the following generations the thankless task of scrambling around recovering enough pieces to mount a continuing defense in the face of the newest threat.
Of course, that was a rather extreme example. The majority usually have just one invader, two at the most for the unlucky, though sometimes it is a "one at a time" type deal for the really unlucky ones.
And the invaders themselves range from slithering slobbering monsters looking for their next meal, to humanity's dark mirror looking for fresh genetic material (the men want the women, the women want the men and the women... the consistency of that last statement puzzles me, I'll admit), each with their own sciences and approaches.
Then there's humanity and it's attempts at holding on to it's home. Some have known of the threat for years, and have plans in reserve for just the occasion. Others only barely thought of the possibility, and find themselves desperately cramming for the ultimate pop quiz that they find themselves taking.
And that assumes that they didn't have a "private tutor" on hand to help them out, or that they had been warned well in advance before being handed the necessary tools.
Of course, even with the latter example, there is a surprising number of "teachers" that are either incompetent, intentionally sabotaging its "students", or are just plain unhelpful.
It is quite fascinating.