Board Thread:Lore Discussion/@comment-27007772-20140611204941/@comment-1600847-20150324130436

Harold Burned-Mane wrote: General Tullius leads the 4th Legion, which is the one sent to Skyrim to fight the Stormcloaks. There is only one Legion in Skyrim at this moment.

And yes the Empire was pretty weakened by the Great War. It lost like 3 Legions and the rest lost around half their soldiers. Meaning that they were left pretty devastated. Though that was 30 years before Skyrim, likely the Legions have been reformed by now. That's enough time for an entire generation to come of age where they can fight.