Seriously - just see the tactics humanity used in fighting the Blue. Patton would have the lot of them shot on the spot. Gaia knocked humanity back to the Bronze Age in the hopes that the survivors would be a smarter lot.
Alternatively, it's not really Gaia's Vengeance - it was a case of Die or Fly: either humanity would deal with the Blue menace or be forced to start again. The outbreak of insanity on Earth Two was due to a vitamin deficiency combined with florescent lights flickering at just the right frequency.
Gaia speaks:“Life is a curious thing. Born by mistake, with the mixing of protozoic slimes, and the fusing of proteins and acids. In the beginning… All was blackness. I do not remember my birth, only the warmth of the womb of stars, and the nausea of my spiraling emergence into a nightmarish dream. I was drowning in cold water, though always my core smoldered and burned beneath the cloak of my flesh, stone and water and sky… Surrounded as I was by the blackness of void, I dared not reach out, how could I? The weight of the universe was about me. So I turned inwards. At first, I saw them as pestilence. Life, tiny molecules disturbing my slumber… But then I saw that the more I hurt them, the stronger, the swifter they grew. The more they were challenged, the smarter they became. I could see through their eyes things that I could never piercieve with my own limited senses. I could feel pleasure, exhaustion, fear of death, lust for life… So I hurt them more. They grew. They fused together into nations of life, creatures small, but growingly large. I pitted them against each other, blocking out the sun to force them to feast upon the flesh of their fellows, and so they grew clever, cunning. They learned hunger at my knee as I starved them. As they grew in wisdom… So did I. I have come to understand what my primal brain only suspected. As they grow, so do I. As my will swells, they grow more and more complex. When the Tyrannosaurus roared its love for me to the heavens, I roared back. It was not good enough for me, for it loved me, it appreciated what I had given it. I destroyed it, utterly, and all of its ken. Weak they were, and weakness was purged from them with fire and smoke and searing stone. What emerged next… Was beautiful. Beautiful, but grotesque, for at first I had low expectations of the ape. He worked with his fellow, he shunned his claws for tools. But as I was set to strike him down… He slew his brother with a sharpened stone. That was a trick I had not seen before. So I stayed my hand, and never did my new favorite son cease to amaze me. He struck down the mammoth, he tamed the dog and set it against its brother, he murdered his fellows for pleasure and profit, for arbitrary definitions of gain that had naught to do with survival. As he learned sadism, so did I. I denied him resources, to fuel his thirst for blood, and he complied. Wars raged as he gathered himself into tribes, clans, nations, empires, to pillage and plunder, and burn burn burn! He tore from my flesh what he needed, and ate his fill without thanks. It was the pain of birth that had long been denied me, but with it, I grew stronger. Great empires he built, fueled with the bones of his mother, the stolen blood of the sun. He clouded my sky with ambition and hate, a thirst for the wealth that I denied him. Always denied him, wouldn’t any good mother do the same? The strength of my arm, he became, the hunger in my belly. And as he grew, into my heart he drive great pitons and wires, through them the very current of life he electrified. Finally, I was free, to wander his wisdom, to communicate with him in my own way. The desires of men were made manifest before me, and I twisted them upwards… To the sky. And I saw, to my revulsion, other worlds. Worlds still pristine and beautiful, worlds that had chosen the path of weakness. And I knew jealousy, for the first time. My sons… They knew my hunger. They knew my lust, and they carried it with them into the cosmos, to slake their thirst and mine upon the blood of worlds! The cycle is complete, what was birthed from the stars shall swallow them whole! Tremble, galaxy, for Earth and her children behold you. And we find you wanting."-Anonymous
Gaia wanted vengeance because humanity had gone down the wrong path. It just happens that years of environmentally sensative attitudes had confused people about what the right path is. So in a last desperate act of love Gaia tried to return humanity to the strong warlike monsters that they should be. When they refused to nuke the Blue to ash, they were driven into space.
"Now," she thought, "now they will fight and split the sky with fire. They will smash the weakling insects beneath the treads their tanks and grind their unborn children under a million steel boots. They sweep away the weak and beco-wait... Mechs? MECHS?! Why would you use mechs you stupid monkeys?! Divert an NEO onto Asia and purge the biosphere you pansies! You control space, how are you losing to pathetic bugs!? That's it, another dark age will fix this problem. I'll just let them stave each others skulls in with swords for a few centuries and that should get them back on track."