The Federation Diplomatic Corps is a body of the Federation that negotiates with other powers.
In 2373, a member of the Diplomatic Corps and an old friend of Curzon Daxs', informed Jadzia that representatives from worlds along the Federation-Klingon border were urging for a preemptive strike. (DS9: "Broken Link")
In 2375, during the Dominion War, the Corps main diplomatic focus was with the Dominion. This meant they were unable to mediate in a border dispute in the Goren system. (Star Trek: Insurrection)