VERY IMPORTANT: TO MAKE SURE YOU DONT MISS THIS ACHIEVMENT, DO THIS BEFORE DOING THE PEASE TREATY FOR THE MAIN STORYLINE... I spent hours for finding out why it wasnt unlocked for me, so i had to load back and start all over...
-To join the Legion travel to Solitude and enter Castle Dour and speak to General Tullis.
-To join the Stormcloaks travel to Windhelm and enter the Palace of the Kings and speak to Ulfric Stormcloak.This achievement is for playing halfway through either side's story-line and capture a fortress. Which is either Fort Sungard or Fort Greenwall depending on who you side with.
Legion Questline;
http://www.uesp.net/wiki/Skyrim:Imperial_Legion#QuestsStormcloak Questline;
http://www.uesp.net/wiki/Skyrim:Stormcloaks#Quests