The Elder Scrolls V: Skyrim Special Edition

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.

Two tips:

1. If the chain bugs around meeting people, just go to the next fort/city and attack solo. Your troops will eventually catch up. I was playing Imperials and was repeatedly unable to find anyone to report to prior to an attack. For the last mission General Tulius was ready sword drawn...but in Solitude. He had no dialogue to start the attack despite being in a combat pose.
I just led a one man raid on Windhelm and the troops caught up.

2. If the achievement doesn't pop, kill something. For me I got it 20 minutes after I finished the chain when I killed a giant fighting my steward. Random but it worked.