By 1943, the term acronym had been used in English to recognize abbreviations (and contractions of phrases) that were pronounced as words. In this view, the modern practice is just as legitimate as those in "proper" English of the current generation of speakers, such as the abbreviation of corporation names in places with limited writing space (e.g., ticker tape, newspaper column inches).
In English, acronyms pronounced as words may be a 20th-century phenomenon. In formal writing for a broad audience, the expansion is typically given at the first occurrence of the acronym within a given text, for the benefit of those readers who do not know what it stands for.
Another driver for the adoption of acronyms was modern warfare with its many highly technical terms. The rapid advance of science and technology in recent centuries seems to be an underlying force driving the usage, as new inventions and concepts with multiword names create a demand for shorter, more manageable names. "YABA-compatible" (where YABA stands for "yet another bloody acronym") is used to mean that a term's acronym can be pronounced but is not an offensive word, e.g., "When choosing a new name, be sure it is 'YABA-compatible'." Acronym use has been further popularized by text messaging on mobile phones with Short Message Systems (SMS).
While there is no recorded use of military acronyms in documents dating from the American Civil War (acronyms such as ANV for "Army of Northern Virginia" post-date the war itself), they had become somewhat common in World War I and were very much a part even of the vernacular language of the soldiers during World War II, who themselves were referred to as G. The widespread, frequent use of acronyms across the whole range of registers is a relatively new linguistic phenomenon in most languages, becoming increasingly evident since the mid-20th century. To fit messages into the 160-character SMS limit, acronyms such as "GF" (girlfriend), "LOL" (laughing out loud), and "DL" (download or down low) have become popular.
Linguist David Wilton in Word Myths: Debunking Linguistic Urban Legends claims that "forming words from acronyms is a distinctly twentieth- (and now twenty-first-) century phenomenon. The capitalization of the original term is independent of it being acronymized, being lowercase for a common noun such as frequently asked questions (FAQ) but uppercase for a proper noun such as the United Nations (UN) (as explained at Case Casing of expansions).