Like your example with Redguard shows, most empires don't win through simple military might, but rather by winning pre-existing local strongmen over to their cause.
Right on. I was just about to say this. Although, this is almost always downplayed by the victors. Take the Conquest of Mexico by the Spaniards in the 1520s. Those relatively unfamiliar with the history will marvel at how Cortez overcame the powerful Aztec Empire with merely several hundred men. Someone who claims this usually explains the victory by talking of the Spaniard's technological superiority, how the Aztecs feared Cortez as a god, disease, etc.
People familiar with the actual history, on the other hand, would be quick to correct this misled person that Cortez conquered the Aztec Empire with (and only with) the help of thousands of Mesoamerican allies, who not only supplied troops to the effort, but also provided vital intelligence and supplies. You see, the Aztecs were hated by pretty much everyone. If Cortez had faced a united Mexico (or Pizarro a united Incan empire, for that matter...) then their conquests, quite plainly, wouldn't have happened.
Bethesda was being sharp here when they created the lore. They knew better than to say "and then the Imperials took their army and pwnzd everybody, and thus founded a great empire." Wars are never won with battles alone.