A timeline for the events of the Generation One Marvel Comics universe(s).
(CONTINUITY NOTE: Beast Wars and Beast Machines take place in a universe that likely shares many facets of this timeline. Though exactly which and how many of these events, or those of the The Transformers cartoon timeline, occur in that continuity is unknown, a link to the Beast Era timeline is provided for reference here at the points where a major historical event from those shows would occur.)
The Dawn of Time
At same point, the beings Primus and Unicron have a punch-up. Accounts differ on the background and the entities involved. One account is they were leaders of the Light Gods and the Dark Armies respectively, leading their forces into battle with each other ; another that they were the last of their pantheons (Unicron's being the Lords of Chaos) and Primus needed to beat Unicron to join the Omniversal Matrix ; another that Primus was called forth by the universe itself as a unique entity to prevent Unicron from destroying it .
Either way, Primus could only win by tricking Unicron into trapping his spirit in a barren asteroid — Primus was trapped in a second. Unicron turned his into a new body, Primus turned his into the planet Cybertron and birthed the Transformers to battle Unicron in his place. The Transformers were led by Prima, the first bearer of the Primus-born Matrix of Leadership.
From an unknown point of time, Cybertron was ruled by the Overlords, a governing council of absolute rulers. Over time, their power and number dwindled, until Cybertron was comprised of multiple city-states who were constantly clashing with each other; the key three powers were Vos, Tarn and Iacon.
In an attempt to lessen growing hostilities caused by the splintering governments and the dwindling resources on Cybertron, the last Overlord started the State Games: an inter-city athletics tournament. This allowed the population to exorcise their hostilities, and soon the vicious athlete Megatron had a cult of personality based around him and his feats.
Vos — ruled by a Starscream-headed criminal syndicate — sent its athletic team to bomb Tarn (ruled by Shockwave) and finger Iacon as the culprit. As with all plans by Starscream, this cocked up — Tarn realised Vos had attacked it, and war broke out that devastated both cities. Iacon happily let its rivals beat on each other. During the conflict, Megatron ensured the death of the Overlord; once it was over, he used his fame and charisma to convince the war's survivors that Iacon was at fault for not stepping in to prevent the war, and that Iacon was the enemy. This angry mass became the first Decepticons.
War on the Horizon
The Decepticons were considered harmless cranks to be humored, but ultimately ignored. Megatron crafted the malcontent group into a more formidable organization , and they soon had bases in Autobot cities. In secret, the Decepticons armed themselves and were given the ability to transform into war machines — and then hit an unprepared Cybertron without warning!
Early Great War
Megatron leads the Decepticons, gaining more and more ground while the Autobots continue to fall; simultaneously, his opposite number Optimus Prime rises in the ranks from squad leader to Elite Flying Corps head to a general, and the two first clash in battle over the powerful Underbase.
Eventually Megatron has pushed the Autobots to Iacon itself, and Optimus Prime leads a successful defensive that turns him back. During that battle, Emirate Xaaron has the Iacon government grant Optimus full control of the armed forces and the ability to do what he wants without needing to go to them — which he was doing anyway!
The war becomes a battle between two evenly matched sides. Megatron, desiring to turn Cybertron into a cosmic dreadnought, builds planet-engines and triggers one that sends the planet off its orbit.  This eventually takes it far too close to an asteroid belt, which will destroy it utterly; Optimus leads a team of the Autobots' finest in the Ark to clear the path of rocks. Following this, he intends to make peace gestures to the Decepticons... who then attack in force and board the Ark, causing Optimus to crash the ship into some uninhabited planet to end Megatron's threat.
The Autobots are left distraught and leaderless by the loss of the Ark, and send Navigator to find it (he gets lost) . The Decepticons quickly gain a new leader in the brutal Trannis, and the war begins going their way once again. Eventually Trannis just gets bored and has Iacon bombed to slag. The Autobots only survive due to Emirate Xaaron having prepared for the army to go underground, and a long-lasting insurgency begins. The commando group the Wreckers kill Trannis, but this just leaves Straxus in charge to start chucking all Autobots, neutrals, empties, failed Decepticons and the postman into the Smelting pool.
Meanwhile, a bunch of Decepticons unlock the key to biomorphic reproduction, a pre-Matrix way of reproducing. This new second generation of Decepticons, including Jhiaxus, happily throw themselves into the war but eventually become bored with it; they leave Cybertron behind to colonise space (exterminating any organic life on said colony worlds in the process) under the aegis of the Cybertronian Empire. Neither the Autobots nor Straxus' Decepticons notice this, carrying on a war Jhiaxus and Co believe is irrelevant.