Sometimes these acronyms' expanded meanings made sense, but most of the time they were words incongruously crammed together for the mere purpose of obtaining a catchy acronym, traditionally a heroic sounding one for the good guys and an appropriately menacing one for the bad guys.
They were presumably inspired by SMERSH, which appeared in the James Bond stories and sounded fictional, but really was a branch of Soviet intelligence.
These acronyms are often spelled with periods/points/stops to make it clear that they stand for longer terms and are not simply the usual English words that they resemble, even though the punctuation would otherwise seem to indicate that the abbreviations should be pronounced as the names of the individual letters.
Among the most popular: Various fiction invent British spy agencies with "MI numbers" other than the well-known MI5 or MI6.
These agencies generally have no relation to the real but defunct branches of the Directorate of Military Intelligence that previously used these designations.