"Die like your Brother!" "They were your brothers too!" Ok as we know the Fallen did not kill his fellow Primes of his generation. The Fallen seems to be afraid of descendant Primes like Optimus. The Fallen said brother not brothers. I'm going out the limb here but I theorize Optimus is not the first later generation Prime the Fallen fought. It was this fight that the Fallen realized the threat of Primes. There is one other Prime named in ROTF, Sentinel Prime of an expedition in Sam's rant. Jetfire certainly has fought Autobots before but does not seem to be aware of Optimus Prime. Lets say Jetfire did not participate in the civil war on Cyubertron but was part of a war in space. The Nemesis seems to have crashed on a moon. How did it get to that state? What if Autobots under a Sentinel Prime under the cover of an expedition fought the Fallen's Decepticons. Course Sentinel Prime died but it made the Fallen realize his own mortality when battling a younger Prime.