(covers information from several alternate timelines)
World War I, also referred to as the first World War, was a conflict fought mainly in Europe on Earth during the early 20th century. In this conflict, the Central Powers, including Germany, fought the Entente, including France and the United States of America.
After Jonathan Archer restored a damaged timeline, scenes from World War I could be seen in the time stream as the timeline realigned itself, which included the Renault FT tank. Also witnessed by Archer was a contemporary scene of American President Woodrow Wilson inspecting his troops. (ENT: "Storm Front, Part II")
In 1930, Edith Keeler asked time traveler James Kirk if he and Spock had served together in the war, which could have served as a possible explanation of why Spock was addressing him as Captain. (TOS: "The City on the Edge of Forever")
Various equipment historically used in World War I, including the French Renault FT tank, the American 75 mm Gun M1916 field gun, and the German Fokker D.VII fighter aircraft, have been utilized in the mirror universe. However, it remains unclear if World War I was also fought in the mirror universe or if those weapons were used in an entirely different conflict. (ENT: "In a Mirror, Darkly" opening credits)
|Earth wars prior to the Federation|
|Crusades • American Revolution • American Civil War • World War I • World War II • Brush Wars • Earth Cold War • Eugenics Wars • World War III • Earth-Kzin Wars • Xindi incident • Earth-Romulan War|
The original script for "The City on the Edge of Forever" featured a World War I veteran; however, this back-story was dropped from the character. Another cut script reference, from "Yesterday's Enterprise", mentioned the assassination of Archduke Ferdinand in Sarajevo, which contributed to the outbreak of World War I.
Spock's figure of six million deaths falls short of the modern estimate of sixteen million casualties of World War I, but may be a reference to the seven million civilian deaths or the number of deaths as a result of "despotism" or be the result of 23rd century historiography.
John Gill wearing a World War I Iron Cross mirrors Adolf Hitler's decoration with this award for his military service during the war.
Hermann Göring, who was featured in "Storm Front, Part II" wearing a World War I style officer's cap, had been a German military aircraft ace in that war with 22 confirmed air-to-air combat kills to his name. Göring went on to head the Luftwaffe in World War II.
A page of the Picard family album created for Star Trek Generations featured a portrait of an ancestor of Jean-Luc Picard in a World War I-era uniform. This portrait actually depicted French army Marshal Ferdinand Foch, which itself was a copy of an actual contemporary postcard, disseminated in 1918 on the occasion of his promotion to Marshal of France and elevation to allied supreme commander.  This portrait did not appear in the finished version of the movie, but was later included in the "Picard's Family Album"-special feature included on the 2004 Star Trek Generations (Special Edition) DVD release.
Performers Paul Fix, Ian Wolfe and Art Director Franz Bachelin are the only three known Star Trek affiliated cast and crew who had been veterans of the First World War, with Fix and Wolfe having served in the American armed forces, whereas Bachelin, like Göring, had served as a fighter pilot for the German side. Star Trek creator Gene Roddenberry was the son of a World War I veteran.
While occasionally referenced to in some Star Trek productions, moving World War I footage was only seen in the Star Trek: Enterprise season four episode "Storm Front, Part II" and in the opening credits of "In a Mirror, Darkly" and "In a Mirror, Darkly, Part II", depicting the mirror universe. With the exception of the President Wilson scene, none of the other footage was actual World War I footage though, instead all of it taken from the 1927 silent – as in no spoken dialog – aviation war movie Wings. Shot on location near San Antonio, Texas, the ground battle with the Renault tanks, was intended to depict the 1918 Battle of Saint-Mihiel, with the soldiers seen having been actual service men from the 2nd Infantry Division (which had in effect taken part in the battle) and the Texas National Guard, on loan by Army and Guard to serve as extras on the movie, 3,500 in total. 
A Paramount Pictures motion picture, produced against a budget of US$2 million ($28.3 million in 2018 prices), Wings was a big success for the studio and considered a technical triumph at the time for its convincing aerial combat scenes – setting the standard for such films for decades to come – , going on to become the very first film to win the "Best Picture" Academy Award at its first 1929 ceremony, and the only silent film to do so as talking motion pictures were by now making the silent ones obsolete, almost overnight. In addition, it also won the very first "Best Effects, Engineering Effects" Academy Award, the later "Visual Effects" category,  for which Star Trek was later nominated, though not winning, thrice (Star Trek: The Motion Picture, Star Trek and Star Trek Into Darkness), and excelled at in the Emmy Award television counterpart.
In 2012, a restored, extended and remastered DVD and Blu-ray Disc version of the film was released on the occasion of the studio's 100th anniversary and the film's 85th, and on which Star Trek alumnus Ben Burtt had worked as restoring sound effects editor.  As with so many other silent-era films, a complete theatrical cut of Wings was actually believed lost as well, until a complete print was uncovered in 1992 in a French film archive. Running for 144 minutes, the film was for its original 1927 theatrical release cut back to a run time of 111 minutes. A copy was quickly made from the fragile deteriorating nitrate film to safety film stock for save-keeping,  and which was the version that saw a 1996 VHS release, the film's second home video release,  after the very first one released by Paramount Home Video in 1985  (reissued in 1989 ), which featured a differently cut version of the film. Deemed "culturally, historically, or aesthetically significant" by the Library of Congress in 1997, a full preservation project was subsequently undertaken in 2002,  the result of which having enjoyed a few showings at aviation conventions in 2007 before its limited theatrical re-releases in 2012 and 2017 (the film's 90th anniversary), having in the mean time been remastered for the underlying home media formats as well. . When Paramount embarked on the remastering project based on the 1996 VHS version of the film, they made use of spare original theatrical release negatives still in their possession, identifiable by being ochre-toned black and white, intertwining it with the cut footage rescued from the French print, identifiable by being blue-toned black and white. The Star Trek-used clippings with the artillery pieces and the Renault FT tanks were trimmed though from the already extensive with French print footage enhanced Battle of Saint-Mihiel sequence on the remastered release, but were originally included on the 1985/89 VHS versions,  from which the Enterprise producers lifted the clippings for use in the series, cleaned-up and hue corrected in post-editing to match the rest of their respective chronology sequences.
Wings director William A. Wellman incidentally, was a decorated World War I veteran himself, having been a fighter pilot in the French-commanded Lafayette Flying Corps – and thus an adversary of Bachelin and Göring – , and with six confirmed combat kills a fighter ace. His war experiences served him well when making his film.  A prolific film director, Wellman went on to direct fellow World War I veteran Paul Fix in his film Island in the Sky (1953) and in his last 1958 one, the aptly named World War I film Lafayette Escadrille (aptly, because it had historically been part of Wellmann's own Lafayette Flying Corps, he therefore coming full circle), both being aviation films as well coincidentally.