Ok, the last Quest of the Civil War - Imperials. Fast travel to Windhelm Stables and speak to Tullius. Afterwards, head inside Windhelm. There's really not much to say about this part. All doors are master locked, so it's probably even a waste of time and lockpicks to try entering any of 'em unless you're good at lockpicking. There are obviously Stormcloaks galore here.
Make your way to the north side of the city while destroying barricades and killing Stormcloaks. Tullius is alongside you, along with Rikke, so its impossible to lose. The direct route is blocked off, so you'll need to take a more round-about path to the left to reach the north side of the city. It's fairly linear and the path will have barricades thrown in at choke points. Once you head to the north side, enter the Palace of Kings...
Note: That if you don't enter the Palace of the Kings, Stormcloaks will still come out and won't stop . YOU NEED TO GET IN THERE!
Once Galmar is taken care of, attack Ulfric with all you've got. Not a hard battle at all. Now, time for a decision: Either you, Dovakiin, kill Ulfric, or let Tullius do it. Pick whichever you want, as it really doesn't matter.
Once you get in here, Rikke and Tullius start talking to Ulfric and Galmar. After some chit chat, you'll fight them. Take out Galmar ASAP since he's more dangerous than Ulfric. Ulfric WILL use his 'Unrelenting Force' shout during this battle, which is entertaining.
Now, you get to keep Tullius' sword and the quest will end, as well as the Reunification of Skyrim Quest. Tullius will head outside and give a speech, so go listen if you wish. Congrats on ending the Stormcloak rebellion!
Hero of Skyrim
Capture Solitude or Windhelm