Operation Retrieve was the code name of a mission proposed by Starfleet Colonel West upon the arrest of Captain James T. Kirk and Doctor Leonard McCoy for the assassination of Chancellor Gorkon in 2293. It was devised, in light of increasing tensions between the United Federation of Planets and the Klingon Empire, to extricate the officers from their captivity by military force.
The operation Edit
The details of the operation involved a direct incursion by several starships, plus several teams of Starfleet Marines, launched from Starbase 24 to the penal world of Rura Penthe, well within Klingon territory. The operation was expected to last no more than 24 hours with an acceptable loss to equipment and manpower.
The plan was vetoed by the Federation President as it would have most likely provoked the Klingon Empire into a full scale war. The operation was subsequently proven to be unnecessary, as the USS Enterprise-A entered Klingon space without authorization from Starfleet and retrieved Kirk and McCoy from the planet. (Star Trek VI: The Undiscovered Country)
Background information Edit
The Operation Retrieve briefing material had a fourth page that was never seen on-screen, and is reproduced to the right. 
"If I recall correctly, the charts visible on film/video listed only ship names and registry numbers. One can probably glean some class designations from the ship icons in the diagrams. I don't have the original art handy (I think it's archived on Syquest disks, which I don't have the ability to read, even if I could find the disks themselves), but I recall giving the info to Bjo Trimble, and I'm pretty sure she used most of it in her revised Star Trek Concordance. I might note that some of the ship registry numbers came from Greg Jein's interpretation of the starship chart in Commodore Stone's office in "Shore Leave" (TOS). Other registry numbers came from Franz Joseph's Star Fleet Technical Manual or his Starship blueprints. In still other cases, the ships and/or numbers did not come from either source, but were consistent with some fleet status charts I did elsewhere on the USS Enterprise-A in Star Trek VI. (In other words, there's something that just about everyone will disagree with, but I also hoped that there would be at least something that almost everyone would agree with.) I should also point out that I prepared several charts for the rescue briefing scene, and that not all of them ended up in the final cut of the film. I don't recall which ones were used, or which ones ended up unseen. I do seem to recall that there was at least one chart that had quite a number of registries - mostly, I recall, from FJ's work - that ended up unused." The names listed in the roster of commanding officers, excepting that of Sulu, were all but one those of production staffers who had been working on The Undiscovered Country at the time. The one exception was of a personal nature, as Okuda included a Captain D. Tathwell, which was the maiden name of his future wife Denise, who, at the time, was the one not yet working for the live-action franchise.
Okuda's remark regarding "Jein's interpretation" concerned the latter's influential "The Case of Jonathan Doe Starship" article which Jein had written years earlier for a fanzine and in which he had postulated a Constitution-class starship registry numbering system. As Okuda has since then strictly adhered to Jein's interpretation, it was a stark indication that he had only became aware of Jein's article during the time he was working on The Undiscovered Country. At the time, Jein had by then become the regular studio model vendor for Star Trek: The Next Generation, befriending Okuda, and was cooperating with him (as was his future wife Denise) on his reference work, Star Trek Chronology, which was concurrently in development. Strictly adhering to Jein's article from then on for those registries Okuda had not yet used at that point in time, this became the very last recorded use of Franz Joseph's Manual.
The Operation Retrieve briefing is, so far, the only canonical mention of the Starfleet Marine Corps. They are not spoken of in dialogue, but are mentioned on the display that Colonel West (who, given his rank, is presumably a SF Marine himself) shows to the assembled gathering.