Optimus Prime is the awe-inspiring leader of the Autobot/Jedi forces. Originally a mere civilian, he was chosen by the Matrix to command, the first in a number of heavy burdens he has been forced to bear. Another is his bringing of the Transformers' conflict to Earth. The complete opposite of his mortal enemy Megatron, every casualty, human or Cybertronian, weighs heavily on his spark. He does not show this side to his soldiers, and he never succumbs to despair. The Autobots need a decisive, charismatic leader, and that is what he gives them. It was that leadership which turned the tide of the Great War.
Transformers: Clone Wars
While fighting Megatron in the Ark outside of a dimensional singularity (similar to the transwarp portal used in Beast Wars), Prime attempted what no Cybertronian before or after ever did. By piloting the Ark into the singularity, the ship crashed on the planet of Muunilist. A few thousand years later, clonetroopers and droids began to fight the Clone Wars on the planet, reawakening Teletraan-1. Low on Energon, the Ark scanned local military hardware, reconfiguring both Autobots and Decepticons into Republic and Seperatist hardware.
Optimus Prime (now in his AT-TE form) led the Autobot survivors of the crash outside, where they saw the Jedi and clonetroopers in the war. Optimus advised the crew on transforming into their robot forms, saying that it would be a bad idea to alert them to the existence of Cybertronian life-forms. However, Optimus himself transformed to save the life of Obi-Wan Kenobi, using a piece of scrapmetal to block a deadly missile.
With the Republic alerted to the existence of the Autobots, the Autobots, Anakin Skywalker, and Obi-Wan Kenobi (along with survivors of the battle) returned to Coruscant and reported to the Jedi Council. Optimus found a similar spirit in Yoda, who appreciated the value of sentient life as much as he did.