It started with the Decepticon uprising that overthrew the Senate. I do not denounce them for that - it was an inevitable conclusion with their restrictions and upholding of distriminatory laws. The first fights were done by miners and gladiators, and others joined their ranks.

The Autobots were the security force for the Senate. Now, their first reaction was to quell violence, but they were not so popular. Unfortunately, after the initial revolution, the disagreements on how to move forward propelled a planet-wide division between the two factions. It quickly devolved into maintaining control and destroying the other faction, with ruling Cybertron as a secondary issue.

[And that's a summary of only the very early history.]

Everything spiraled out of control for the next four million years.