This page contains information regarding Star Trek: Discovery, and thus may contain spoilers.
The Federation Diplomatic Corps was a body of the Federation that negotiated with other powers. Members of the Diplomatic Corps were present at the Khitomer Conference. (Star Trek VI: The Undiscovered Country)
In 2372, a member of the Diplomatic Corps and an old friend of Curzon Dax's, 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. As a result of this, the USS Enterprise was relegated to a diplomatic role, to the dissatisfaction of her captain, Jean-Luc Picard. (Star Trek: Insurrection)
In 3189, Fleet Admiral Charles Vance contacted the diplomatic corps and gave them the news he was sending Commander Michael Burnham as a Federation representative to Ni'Var, with the hope of restarting diplomatic relations with this planet. (DIS: "Unification III")