Starfield:Diplomacy

Diplomacy is unlocked by spending a total of 4 skill points in the Social skill tree. It allows you to convince NPCs to stop fighting. In order to use diplomacy, you must target an NPC with your scanner, then open the social menu, then select "Diplomacy". Diplomacy has a chance to fail, and once you have failed to use Diplomacy on an NPC, you cannot use it on them again. It will not work at all on NPCs that greatly outlevel you.

Diplomacy also unlocks new options in Speech Challenges, which are guaranteed successes, making persuasion easier.

Skill Increases
The fastest way to quickly progress through the skill rank challenges is to repeatedly use the Diplomacy action on friendly NPCs.