20th Century Fox's Alien franchise has been successful for more than three decades, with plenty of tie-in movies, games and merchandise to its name. Yet, there were also plans at one point to market a cartoon series based on the films!
Operation: Aliens was a proposed cartoon pilot that was developed in 1992, both to coincide with the release of the third film in the series, Alien 3, and with a proposed line of action figures Kenner Toys was producing. Details on the pilot itself are scarce, but what is generally known is that the idea to do a cartoon series was envisioned by Fox during the marketing blitz for the third film, and work was outsourced to a Korean animation studio.
What happened next is unclear. Production evidently got far enough along that an Operation: Aliens toy and merchandise line was planned and designed, and that the pilot was due to be aired in Fall 1992 on Fox's television network. However, the pilot was scrapped, although late enough in the process that several pieces of merchandise went to store shelves still bearing the Operation: Aliens banner.
Assuming that the plots outlined in the mini-comics from the Kenner toyline were intended to be part of this cancelled series, the pilot would have followed Lieutenant Ellen Ripley and several returning members of the Colonial Marines (including deceased ones oddly enough) as they captured or destroyed dangerous Xenomorphs who were attacking various locales. One major conceit of the action figure line is that the Xenomorphs were breeding new hybrids, in the form of the Chestbursters assimilating various traits of their hosts (including a bull, a mantis, a scorpion and others).
It's likely the pilot was scrapped when members of the production realized that it's incredibly difficult to plan a children's program where the antagonists are double-jawed, phallic-shaped creatures with acid for blood. It's also likely that the lackluster box office performance of the third film put a damper on any future opportunities for tie-in films or shows. A popular theory may be that the Action for Children's Television deemed this to violent and saw it as a TV commercial for a line of toys which TV activists was strongly agent and which the government was making a law on around that time. and selling a movie that kids are too young to see ,There for marketing a product to the wrong demographic.
The pilot has never been released in any form, and the only evidence that it was at least partially animated are a series of screencaps that appeared on a now-defunct website for Korean animation.
The toyline was eventually released under the name "Aliens" (after the film from 1986). The toys lasted until 1995.
UPDATE: As of a December 14th 2018 article written by Corporal Hicks on the Aliens Vs Predator Galaxy fansite, the existence of this cartoon has been proven false as former Fox Kids executive, Margaret Koesch, explained that the network never even ordered a pilot episode nor did the idea of an animated kids show originate from the Korean animated studio. What is true though is that Kenner approached Fox with the idea of doing a series to coincide with their action figure line, but was shot down due to concerns over broadcasting standards along with Fox’s theatrical division protesting over “turning Aliens into a children’s property.” As for the animation screen grabs that some people claimed were all that remained of the long lost pilot, they are actually the work of Will Meugnoit from an unaired commercial he produced with AKOM Production in 1992 and the Internet, as usual, pulled them to start some urban legends. Many of the animated commercials for the Aliens toyline are also lost as they were drafted in favor of live action advertisements showing kids playing with the figures.