The United Earth Space Probe Agency, commonly abbreviated UESPA, was an organization centered on Earth responsible for the exploration of space. They were headquartered at UESPA Headquarters. (TOS: "Charlie X")
In the 2260s, UESPA was still a major operating agency working with Starfleet vessels. By 2267, UESPA was the operating authority of the USS Enterprise's five-year mission, as Captain James T. Kirk informed John Christopher. (TOS: "Tomorrow is Yesterday") In 2266, Kirk reported to UESPA Headquarters after learning of the destruction of the Antares. (TOS: "Charlie X")
USS Enterprise (NCC-1701-B)
UESPA (pronounced "you-spah") was first mentioned in abbreviated form in TOS: "Charlie X" and the acronym was spelled out in TOS: "Tomorrow is Yesterday". It was one of several names given in early episodes for the agency under which the Enterprise and its crew operated before writers finally settled on "Starfleet".
UESPA was mentioned in unseen portions of the Picard family album, in the article "It's Federation Day!". Major General Georges Picard, an aide to Thomas Vanderbilt, implies here that he fought the Romulan scourge, which itself implies that UESPA was in some way involved in the Earth-Romulan War.
Captain Kirk refers to UESPA as a "combined service," but he is vague enough that it is more likely that he was actually referring to his Starfleet crew. It is possible that starship crews of his time were a combined service of both the naval ranked Starfleet and the military ranked UESPA officers. This would explain the use of military ranks in "The Cage". This may also explain West's rank of colonel in Star Trek VI: The Undiscovered Country.