Military Ranks are military grades, which depend on the "hierarchical organization" of the Military.
To get advanced in military rank you should improve your ranking points, which you can get by:

- Defeating enemies
-killing troops from the rival server in Cross-server border war event.
- Defeating troops defending rebel warehouses from any server.
- Killing enemies in your own server will get you fewer ranking points, and if the enemy's Command Center is of a lower level than your own. This only applies to enemies in your own server.