The overwhelming majority of the Star Wars franchise is based around noble duels between the series' many magic-wielding knights. Though every Star Wars film has taken place in the midst of an intergalactic conflict, there's only one that put that element of the franchise front and center.
When someone refers to a war movie, they're usually picturing something akin to Saving Private Ryan, Apocalypse Now, or Dunkirk. The war, real or fictional, serves as the setting, character, and overarching plot. It's the backdrop for the characters as they live out their personal stories. It's this distinction that makes the genre different from the action or horror films they often resemble.
Rogue One: A Star Wars Story takes place immediately before the original Star Wars trilogy, ostensibly to offer the full backstory of a single line in A New Hope. The film tells the story of a ragtag team of underdogs banding together on a suicide mission to steal the Death Star plans from the Empire. Rather than the typical cast of heroes, the Rogue One team consists of lifelong rebel Cassian Andor, reluctant ex-convict Jyn Erso, blind warrior Chirrut Imwe, heavy gunner Baze Malbus, and a robot called K-2SO. Together, this group of freedom fighters enacts assassinations, armed raids, and full-field combat with armies of stormtroopers.
In Rogue One, the battle scenes are the highlight of the film. They're large-scale action scenes with countless soldiers, lasers flying in all directions, and X-Wings passing overhead on bombing runs. The level of battlefield focus and the specific filmmaking decisions change the impact of the conflict between the Rebels and the Empire.
There are certainly battle scenes throughout the rest of the Star Wars franchise. Memorable ones include the struggle on Hoth in Empire Strikes Back, the battle on Naboo in The Phantom Menace, and the conflict on Crait in The Last Jedi,. The difference is that these scenes aren't the focus of the films — they're set-piece moments between the real draw. The wars in Star Wars are important, but the fine details aren't the focus of the main films. The camera mostly stays on the handful of important characters that everyone knows. The big plot moments come from the lightsaber duels between whichever Skywalker family representative is up to face this generation's Sith lord. It's about chosen heroes taking on fated villains for the fate of the galaxy. This is more classic action movie storytelling, and the difference can be explored through tons of great cinematic examples.
For a comparison, consider Saving Private Ryan and Captain America: The First Avenger, which both take place during the Second World War. Obviously, Marvel's effort heavily fictionalizes the global conflict, but both movies depict elements of World War II. Saving Private Ryan is first and foremost a war movie. It's about the movements of the various forces on the battlefield, as seen through a few important and identifiable characters. Captain America is an action movie, that is set during a war. It's about the journey of Steve Rogers as he becomes the eponymous hero and defeats the evil Red Skull. It would be very out of place if Saving Private Ryan ended with Tom Hanks and a Nazi Commandant engaging in a one-on-one duel to the death. It would be equally weird if Captain America ended with the American infantry pushing back the Germans as Steve learns a valuable lesson about sacrifice. The difference is a bit questionable, but can be determined.
Rogue One definitely features key characters, but it is not really their story. It's the story of the Rebel forces leading the series of military actions that result in the theft of the Death Star plans. Larger trends and forces are at play, and the film occasionally looks in on the leadership of both sides. Even the Empire is on display from time to time. There is very limited focus on backstory, and most of what's there is delivered through dialogue. The stormtroopers are, as is often the case, seen as disposable pawns in the battle, but the film also has Riz Ahmed's character. Bodhi Rook defects to the Rebel alliance, a very rare occurrence that changes the face of the Empire. The clandestine activities, broad focus on the battlefield, and unique approach to storytelling make this film distinct from its franchise peers. But one of the biggest details in this shift actually comes in the film's end.
Rogue One ends with the success and immediate vaporization of every hero in the film. This is probably the darkest film in the Star Wars franchise overall. Cassian Andor has a strong moment wherein he reveals his difficult history on the battlefield and life in the struggle. Of course, they can't resist shoving in some classic Star Wars bad comedy, but most of it comes with a much harsher tone than the main trilogies. This is common in war movies. "War is hell" may as well be the subtitle of every film in the genre.
Rogue One was far from perfect, but its tone, characters, and narrative set it apart from the rest of the franchise. Star Wars should expand its genre more often — maybe other experiments will work out better.