Reminder Tullius was only meant to be in Skyrim for a few months and was not equipped for a war
Reminder the "Imperial Legion" in Skyrim isn't the actual Cyrodilic Legion, but rather locally recruited Nords and farmers and stuff
Reminder the Stormcloaks are scared shitless of the actual Legion showing up from down south and they're not equipped to deal with it
https://en.uesp.net/wiki/Skyrim:Stormcloak_Missive_(Fort_Neugrad)
Reminder even with all of the above, the Empire had still won the civil war by the time the game starts and it took a literal deus ex machina dragon god appearing outta fucking nowhere for Ulfric to not be beheaded right there and then

Kneel