The first thing Tony Stark noticed was that his instruments were lying to him.
The HUD inside the Iron Man helmet painted the Chicago skyline in clean blue lines, but the readings underneath were screaming numbers that didn't belong to any aircraft, any missile, any reactor he'd ever scanned. Mass like a bridge. Power signatures like a buried sun. And it was moving.
"JARVIS, tell me that's a glitch."
"I'm afraid not, sir. The objects are biomechanical. They appear to be... walking."
Below him, a thing the size of an office tower unfolded out of a crashed ship, all blades and burning eyes, and gutted a skyscraper as if peeling fruit. Glass rained down forty stories. Tony's stomach dropped with it.
"Okay," he breathed. "That's new."
The quinjet came in low over the river, Steve Rogers braced in the open hatch with his shield magnetized to his back. Natasha had the stick. On the comm, Director Fury's voice was clipped and cold.
"Be advised, the city is under occupation by a hostile non-human force. We have civilians trapped on Wacker Drive. And Captain, we have a second variable. There are two factions out there."
"Two?" Steve said.
"Some of these machines are fighting the others. We don't know whose side they're on. Don't assume any of them are friendly."
Steve looked out at the burning city and set his jaw. "Understood."
They dropped him on a rooftop near the lake, and the war was already there.
A Decepticon, sleek and merciless, had a knot of soldiers pinned behind an overturned bus. Steve hit the ground running, shield up, and put himself between the cannon and the men. The first blast knocked him flat and slid him ten feet across the gravel, but the shield held, ringing like a struck bell.
Then a yellow car roared up the ramp, transformed in mid-leap into something tall and golden, and slammed the Decepticon off its feet with a fist like a wrecking ball.
The two giants grappled. Steve scrambled the soldiers down the stairwell, then turned back, expecting to have to fight both. Instead the yellow one threw the gray one into a parking structure, looked down at the small blue figure on the roof, and made a noise like a radio scrubbing through stations.
"You and me, then," Steve said, and didn't lower the shield.
A voice answered from somewhere behind the yellow machine. Human. A young man, sprinting and out of breath. "Don't shoot him! He's a good guy! That's Bumblebee, he's an Autobot, he's protecting people, the same as you, I swear."
Steve looked from the kid to the giant. The giant offered, very slowly, a hand the size of a sedan, palm up. A gesture older than language.
"Captain," Fury's voice crackled. "Status."
Steve stared at that open hand. "Sir," he said. "I think we just found out whose side they're on."
Across the city, Tony was having a less diplomatic introduction.
He'd been trying to evacuate a stalled El train when something enormous landed in front of him, three stories of weaponized contempt, and addressed him directly.
"A flesh creature in a metal shell," it sneered, in a voice like grinding tectonic plates. "You imitate us. How quaint. I am Megatron. This world belongs to the Decepticons now."
"Yeah, see, that's a problem," Tony said, "because I just put a down payment on a building here."
He fired everything. The repulsors splashed harmlessly off armor older than the human species. Megatron swatted him out of the air, and Tony went through a billboard and into a fountain, the suit screaming warnings.
"Sir, the armor cannot sustain that level of impact," JARVIS said.
"Noted," Tony coughed, hauling himself up.
A truck horn blasted across the plaza. Blue and red, chrome and flame, a semi rig folded upward into something vast and noble, a sword sliding from its forearm in a single brutal motion.
"Megatron," the new giant said, and the word carried more history than Tony could imagine. "You will leave this planet, or you will die on it."
The two titans collided. The shockwave flattened parked cars. Tony staggered backward, watching the most absurd and the most magnificent thing he had ever seen.
"Who," he managed, "are you?"
The red one parried a blow and answered without looking away from his enemy. "I am Optimus Prime. We are autonomous robotic organisms from the planet Cybertron. We did not bring this war to your world." A spinning kick caved in Megatron's chest plate. "But we will end it here."
Tony's HUD lit up with the energy signatures of dozens of Decepticons converging on the city center. He keyed the team channel.
"Everybody, listen up. The big walking buildings come in two flavors. The ones with the murder vibe are the bad guys. The ones who keep saving us are the Autobots, and we are officially on their side, retroactively, effective five minutes ago."
"Stark," Fury cut in, "you don't have authority to make alliances with extraterrestrial military forces."
"Then court-martial me later," Tony said, lifting off. "Right now I've got a friend named Optimus and he needs an air force."
What followed, the survivors would describe for years and never quite be believed.
Thor came down through the cloud deck like a falling star, Mjolnir trailing lightning, and met a Decepticon flier in the air. Hammer met hull. The thunder rolled off the lake. "You wear iron and you have no honor," Thor declared, riding the machine down into the river. "On my world, even the constructs of the dark elves fight with more grace than this."
The Hulk did not need a strategy. He needed a target the right size, and for the first time in his life, Chicago was full of them. He leapt from rooftop to robot, ripping armor plate, roaring into faces that had terrified entire galaxies, and the Decepticons, for one beautiful moment, looked afraid.
Hawkeye's arrows found optical sensors and joint seams that Bumblebee marked for him with a targeting laser, the two of them working a corner of the battle in perfect, wordless rhythm. Natasha rode the shoulder of an Autobot named Ratchet, calling out civilian positions while he covered her with field fire.
And Steve Rogers fought beside Optimus Prime.
It should not have worked. A man and a god of war, one the size of the other's hand. But Steve had spent his whole life being smaller than the fight, and he knew exactly where a smaller thing could matter. He drew fire so Optimus could close the distance. He pulled a wounded Autobot's human partner from a collapsing lobby. When Megatron got a hand around Optimus's throat, it was Steve's shield, hurled with everything in him, that struck the warlord's optic and bought the second that Optimus needed to drive his blade home.
Megatron fell, and the sound of it was the sound of a war ending.
In the smoking quiet afterward, Optimus Prime knelt, bringing his great face level with the small soldier on the rubble.
"You did not know us," the Prime said. "Yet you stood with us."
Steve wiped blood from his lip. "You were protecting people who couldn't protect themselves. Where I'm from, that's the only side worth being on."
Tony landed beside them, faceplate sliding up, smoke curling off his shoulders. He looked up at Optimus for a long moment, then stuck out a scorched gauntlet, ridiculous and earnest.
"Tony Stark," he said. "I, uh, build the small imitations. We should talk. I have so many questions and a workshop with terrible parking."
Optimus regarded the offered hand, then touched it, one metal finger to the whole of Tony's arm.
"Freedom," the Prime said, "is the right of all sentient beings. Today, your kind and mine learned we share more than a shape."
Overhead, the smoke began to clear, and the impossible alliance stood together in the ruins, watching the sun come back over a city they had saved twice: once from the sky, and once from the easy mistake of fearing what they did not yet understand.