This achievement can be obtained by completing the sidequests of the civil war for Skyrim.
There are two ways to be done:
1. Follow the questline of the Stormcloaks to capture Solitude, or
2. Follow the questline of the Imperials to capture Windhelm.
It does not matter who you allied with on the first mission. What matters is which of the questlines you follow.
It does not matter who you favored in the Greybeards War Concil, but your choices can change the questline.
To join the Stormcloaks, travel to Windhelm and speak to Ulfric Stormcloak.
To join the Imperials, travel to Solitude and speak to General Tulius.
Good luck.