Now return to the Pale Imperial Camp and speak to Rikke once again. This will start this Quest. Fast travel to the nearest location and make your way to the marker. Once you join the Legion soldiers, now you need to defeat the Stormcloaks. Just like in the battle for Whiterun, a % appears. Enemies come out from all the entrances to the Fort, so make sure to use those Legion Soldiers as bait and take them all out.
There's a lot more enemies here than there was in Whiterun; so you'll likely need to heal frequently if you're a melee-fighter.
Now return to Solitude and speak to Tullius and receive your new rank (Prefect) and blade (a random weapon).
Note: If you fled the Fort and left the Legion alone to take it back, you won't receive the blade. Also, Tullius gave me a STEEL SHIELD as a reward! So much for a "blade"...
Now, under the Reunification of Skyrim Quest you'll get a new objective: Regain the Rift. Now we need to go to Rift Imperial Camp, which is west of Riften. Fast travel to the closest location and make your way to the camp. Once you reach this place, speak to Rikke and your new quest will start.