Crusader Kings III

This will be one of the first achievements you unlock in the game; vassals and peasantry can start factions against your rule if they are unhappy or want to press a personal claim. When they have reached the faction unrest threshold, they can send you an ultimatum to give in to their demands or go to war. By the time anyone reaches this point you should have a strong enough military to be able to choose the war option and, should you defeat them (which you likely will), the achievement will then pop up :)

Edit: Please give a reason for a negative vote! Many early achievements are ignored so I wanted to help anyone that may still be stuck xx