The Chituari invade expecting a higher form of warfare. They can read the emissions of the Tesseract, know that Asgard's crown prince has made multiple trips to this planet tucked away in a lonely corner of the galaxy. They expect it to have flourished on its own, insulated from the hindering alien influence so many other worlds have suffered through. They expect isolation means energy to gear towards progress, towards grand inventions and magic the likes of which have never been seen before.
After all, why else would the Asgardians hold the planet in such high regard?
Their initial rush is both a euphoric victory – such an enemy, so easily taken! – and a sound disappointment – such an enemy, so grossly overestimated. The Chitauri wipe their boots on the corpses littering their island foothold and sneer in disgust at how much they had overestimated these weakling natives. They contact command, say to stand down the second wave. Their mission will be finished quickly; they don't need the support. Command sends an acknowledgement.
Then the portal goes out, cutting the invaders' lifelines – their supplies, their reinforcements, their base of operations. Ten thousand voices scream as one in outrage.
The Chitauri expected a higher form of warfare. That's their problem. Humans die easily in war; what they thrive on is conflict. The Chitauri burn every city and march through every ruin, but the Tesseract remains hidden. Their field leader screeches in anger and the soldiers cower in fear.
Reports of sporadic battles filter up the chain of command. An entire platoon disappears in one of the planet's largest jungles. A strange mechanized suit flies over an encampment, breaks through the shielding, and detonates a device that leaves behind only a smoking crater and the shadows of their brethren seared on the ground. A pair of unseen enemies assassinates the scientist in charge of tracking the Tesseract. A man with a round star-emblazoned shield disrupts a raiding party, leading a ragged platoon of humans.
Supplies start to spoil. Soldiers start to disappear. Hardware starts to break.
The Chitauri are a warrior race in the old sense of the phrase. They are meant to fight and destroy, take what's theirs and leave. They don't occupy. It is a strange mentality for them, alien and unknowable. Without the larger ordinances, stored on the command vessel they have lost contact with, they cannot destroy the humans completely. The insurgents are as smoke through their fingers; the Chitauri never have a clear target to strike, and to strike indiscriminately is always a risk. Unless they find the Tesseract, they are stranded on this backwater planet.
Their numbers dwindle. The commander repositions troops, but there are simply not enough to control the planet. Everywhere humans slip through their lines, regrouping and rebuilding. It's a constant war to destroy them. They are struck down but some always survive to regroup, organize, and try again.
Someone is leading them – a human with an eyepatch and a long black coat, and no other description. The humans call this leader Fury when questioned. They spit the blood that bubbles on their lips and yell a rally for Fury, a rally for vengeance and wrath. His generals are warriors the Chitauri have met at the initial incursion and many times afterwards: the green monster, the metal man, the shield bearer, the archer, and the assassin. The commander questions every prisoner as to their whereabouts, but the answers have never borne fruit. Fury is a ghost, and his generals never stay in one place long enough to track.
Furious, the commander decides to track them down himself. He is the strongest of the Chitauri, fearless in battle, genius at strategy and supply.
He comes the closet, but again and again they slip away. The Chitauri don't understand this race. Their values and their logic has developed separately from the rest of the universe, isolated as it has been for so long. The common ground other races depend on for interactions – honor, pride, rules of combat – is absent. They strike and retreat without computation, gather forces and strike again. They are crude and vicious and so very strange.
Every day the troops, stranded in a strange and hostile land with no chance of rescue but to find a cube that has so far evaded them, become more discouraged. Their commander, so strong and glorious in their eyes, has failed his sworn task. He cannot find the generals. Captive humans don't let the Chitauri forget it: in their cells they sing of the Avengers, taunt the guards walking by with the threat of their bogeymen. Continued failure follows discouragement, and so the invading party spirals further and further towards destruction, nipped at the heels by a thousand small skirmishes.
Their reach, tenuous as it was, had once encircled the planet. Now the Chitauri can barely secure a continent. Humans, emboldened, start engaging them on larger scales. The Avengers lead them, battered and exhausted – but alive and therefore already victorious. The numbers of Chitauri dead, once a steady trickle, has turned into a hemorrhage. Finally they retreat back to the island, to the first place they had set foot upon the Earth.
Their food is gone, their technology is broken, their numbers are miniscule. Beyond this last shield generator they salvaged, the last functioning weapons any of them have are tooth and claw. The soldiers dream of portals, of the deep black of space and the safety of superior numbers . The foreign sky burns a clear steady blue day after day, as if taunting them.
And then they come. The Avengers, Fury by their side. Proud, defiant, and victorious. The Chitauri shift, restless behind their protections. These are the bogeymen that have been hunting them since they've set foot on this thrice-damned planet. These are the mightiest heroes of Earth that have beaten every attempt to kill them, that have succeeded against all odds. They are the Avengers, dealing back all the harm done to their people tenfold.
The Chitauri are, though they balk at thinking of themselves as such, afraid.
"You know," the metal man says, "A few years ago the human population was over six billion people." His voice, broadcasted through the speakers on his suit, echoes throughout the desolate wasteland of rock, metal, and crumbled concrete. He looks directly past the battlements to where the commander is standing, and it sends a shiver down his spine. "We were everywhere. We owned this place. And then you guys came along. You invaded, looking for a pretty blue box that less than a hundred of those billions of people had ever seen. That didn't matter, though; you came and you killed, indiscriminately."
The alternate form of the green monster – diminutive but no less dangerous, a hard-earned lesson – speaks up next. "We're not sure exactly how many people survived, but our best estimate is less than two million. That's a lot of blood on your hands."
"That," says the shield bearer, "is genocide."
"You want to know what happened to the Tesseract? It's not here. We sent it off this planet the minute we knew you were looking for it. It's been out of your reach for years and you've been chasing ghosts." A smile twists the assassin's face in satisfaction once she has spoken. It's a smile the commander has seen in the faces of those he'd captured – sharp and defiant, with a secret knowledge of something he'd never been able to pry out.
The archer beside adds, "We sent it away because if we couldn't save the earth, we damn well wanted to avenge ourselves with the death of every last one of you who ever set foot here. You messed with the wrong planet, and we're going to eat you alive for it."
Fury, their leader, finishes: "We would let a few of you live, throw back your bleeding carcasses to tell your commanders what a bad idea it was to fuck with us – but what the imagination conjures is so much better than reality, don't you think?"
The shield bearer hefts his weapon. "Avengers," he calls. Around the battlements of the Chitauri ragged humans rise from the cover of the rubble, armed with a motley collection of bladed and projectile weapons. The metal man fires something at the shield that makes it crackle and fracture. "Assemble."
