This achievement and "May my Hand Forget its Skill" are mutually exclusive. They are the two different endings for the game, and one must be chosen for completion of the DLC. Read on to know when to save in order to refrain from a second playthrough of Honest Hearts.
After completing the "In a Foreign Land" achievement, you will get the quest "Gathering Storms." For this quest, you must talk to Daniel in order to decide the fate of Zion. Save before you talk to him! You can talk to him about the decision and still have the opportunity to back out in order to complete any side quests, so back out of the conversation and save your game. In order to get this achievement, choose the option to fight the White Legs. After choosing the option to kill the White Legs, you will receive the quest "Crush the White Legs." Just complete all of the objectives within the quest, the ending cinematic will play, and the achievement will unlock.
Refer to the walkthrough in "May my Hand Forget its Skill" if you're having problems.