Disclaimer: I own nothing. Transformers is owned by Hasbro. History left out of Starscream's Sparkmate prior to Starscream's Sparkmate. No real slash, but is implied toward the end. This is merely a fan fic.
This starts millennia ago, following Megatron and Optimus.
Optimus sat there drinking a bottle of energon his mom had just given him, cooing softly. Optimus was a quiet, gentle sparkling. His twin, Megatron, on the other hand, was a rambunctious sparkling that always demanded so much attention. Optimus sat quietly on the metallic floor playing with Megatron. And even though they were different as night and day, Megatron and Optimus played together well and even slept in the same crib, as their parents were not expecting twins but could not afford another crib. The twins did not mind as separating them meant hearing no ending cries from Megatron and Optimus cowered in a corner. So the twins were kept together and grew up together.
When the twins were old enough to go to school, they walked to the school together, Megatron always watching out for Optimus. Optimus, the shy gentle bot that he was, generally drew the bullies toward him, but one look at Megatron would scare them away. It was during this young time at school that Optimus met with his first friends, Ironhide, Red Alert, Prowl, Jazz, and Ratchet (who was still known as a cranky bot at that time). Megatron met up with the Seekers, Starscream, Skywarp and Thundercracker, brothers from one family and Megatron also met a very young Soundwave and Shockblast. Each twin was well acquainted with the other twin's closest friends. For some unknown reason, the school had decided to keep the twins separate in different classes. (Later on it would be found out that due to the Lamborghini twins causing trouble when together, thus no set of twins were ever allowed in the same classroom). And so school came and went.
After graduating from the version on Cybertronian high school, Megatron and Optimus were going to go to college when the war broke out. The two twins wanted to stay out of it, but after witnessing their creators/parents get killed, they knew they no longer could avoid the war. Optimus immediately went to the Autobot side, hoping that they could end this war as the Autobots wanted peace, which was Optimus wanted. However, Megatron, thinking that fighting was the only way to go joined the Decepticon faction, hoping there fighting and terrorizing would end this stupid war.
Both bots started very young and both quickly rose through the ranks. Ironhide, Prowl, Jazz, Ratchet, and Red Alert stuck with Optimus, going to the Autobot side. The Seekers, Soundwave and Shockblast, followed Megatron to the Decepticons. Before either of the Twins realized it, they had transformed into battle hardened soldiers, and had reached the opt rank of both sides. It was many years in the making, but the reunion of the twins would not be a good one.
Optimus had yet to reach his Prime status, but he was second in command. Prime's leader had gone out to check on a post that had been reported as being attacked by Decepticons. Prime, realizing the danger followed his leader, who carried the Matrix of leadership, out to the spot of the last sighting.
When Prime got there, he saw his leader down on the ground, facing a nasty looking mech who had just shredded the Autobot Leader. Optimus, battle toughened, went straight for the mech now trying to choke the life out of the Autobot leader. Neither Twin recognized the other and both had their armor remade and neither remembered the bond the used to share. Optimus quickly jumped into the fray, pulling the Decepticon off of the terminally injured Autobot leader.
"How dare you defy the mighty Megatron," Megatron shouted, Optimus failed to recognize the name due to the fact that he was trying to protect the Autobot leader.
"I will teach you. You can't stop me" replied Optimus. He charged at the Decepticon leader and then Optimus felt a pain on the side of his face. Optimus fought his CPU out against the larger bot. Finally, as the Decepticons retreated, as Optimus knelt down to assist the wounded leader.
"Optimus…" the leader gasped "You have been very brave, and since I am going now to join the Matrix, I want to give…you…this." Opening his chest the broken leader revealed the Matrix of leadership. Optimus could hardly believe his eyes, he was merely one soldier doing his job. The dying leader handed the Matrix of Leadership to Optimus, who stood their stupidly, looking between the Matrix in his hands and the dying Autobot at his feet.
"I know...I know…I know you can do it – Optimus Prime" and with that the dying leader's optics went dark, never to come back on. Optimus sat there for what seemed like an eternity, but in reality was just moments, processing what had happened. Other Autobots came running to see what had happened and saw Optimus possessing the Matrix of Leadership. Optimus brought the Matrix up to his chest and opened his chest cavity. When the Matrix connected to Optimus's spark, the small warrior grew his body grew into the form he is in present day.
After returning to the Autobot base, Optimus Prime, as he was now known, thought about the battle with Megatron. He still had the large gash in his faceplate, but he could cover that with a battle mask. What he was pondering, though, was why the Decepticons' leader's name was so familiar. That was when it hit him, the Megatron he had fought was his twin brother, twisted and evil, not at all like the kind loving brother he remembered. Oh no, thought Prime, this is not good.
Meanwhile, at the Decepticon HQ, Megatron was spouting off, agitated that the plan they had had failed so miserably. The one thing he had not counted on was the second Autobot. The Autobot had never said his name, but Megatron felt as though he knew him. Megatron had watched from a distance as the leader he had destroyed passed the Matrix to the young Autobot that had stopped Megatron. There was only one bot in the whole universe who could stop Megatron. No – Megatron thought worriedly – it is not possible. He then realized in horror the truth, the new leader and commander of the Autobots was his own twin brother. Megatron's optics blazed. Fine, he thought, all my family is dead. I alone will save the world by dominating it.
And the war continued - neither side giving an inch.
Frustrated by Megatron and his continuous beatings, Starscream fled and joined the Autobots, where he first met Hotshot, a tough young warrior who would eventually win the seeker's heart. Demolisher, more loyal and younger than Starscream stayed on bored, and began to go after Cyclonus romantically. None involved saw what was going to happen in the future.