To put it simply, a main character's Disappeared Dad or occasionally Missing Mom is absent because they are off fighting in a war. When this trope appears in realistic fiction, it's usually used as an excuse for the parent to be out of the picture without actually being dead, but in adventure stories it's more likely to be an integral part of the story. In historical fiction, said war is likely one of the world wars.
A sub-trope of Parental Abandonment. If the parent returns safe and sound, this may result in Dad the Veteran. Often overlaps with Keep the Home Fires Burning, but it different because the main plot focuses on the heroes having their own adventures while the parent is (usually) less important.
- Miss Kobayashi's Dragon Maid: Both of Kanna's parents are chaos aligned dragons that take part in the war against the harmony faction. As a result, they usually left her to her own devices and are incredibly neglectful. Her father Kimun doesn't even seem to be able to grasp the concept of a father-daughter bond, instead referring to her as a comrade (note that Kanna isn't even aligned with the chaos faction, or any faction for that matter).
- In the 1995 film adaptation of A Little Princess Sara starts attending the boarding school due to her father's enlistment and she eventually receives news that he died in the Great War. The news of her father's death causes the matron of the school to turn Sara into a servant since her tuition is no longer being paid and there's no one to stop her.
- The Others: Grace's husband is absent because he's been fighting in World War II, so she's left to raise her two children by herself. He eventually reappears and spends the night before telling her that he has to "move on" and leaves just as abruptly. This is because everyone is a ghost. He died in the war, while his wife killed herself and their children but is suffering from Ghost Amnesia.
- One of the illustrated short stories in Edward Gorey's book Amphigorey is "The Hapless Child." The child is named Charlotte Sophia, and her father is a colonel in the British Army. He is reported to have been killed in action aboard, upon which Charlotte's mother falls into a "decline" that proves fatal. Now parentless, Charlotte Sophia is remanded to an orphanage, where her life goes From Bad to Worse.
- In The Chronicles of Narnia, the father of the four main characters is absent due to fighting in World War 2.
- In Little Women, the girls' father is away serving as a chaplain in the American Civil War.
- Friday Night Lights: Matt Saracen lives with his dementia-stricken grandmother, Lorraine, while his father, Henry, is deployed in Iraq. It's clear that this has been the arrangement for most of Matt's life. Several times throughout the series we see Matt go to school early to use the school computers and web cams to talk to his dad and this seems to be all the contact he has with either of his parents. As Grandma Saracen's dementia gets worse, Matt begs Henry to come home, but his dad insists that he can't abandon his duties as a soldier. When an incident calls into question Lorraine's ability to continue as Matt's guardian, Henry has to come home to deal with it as Matt has no other family, his mom having abandoned him as a baby. When Henry arrives home, it's clear that he doesn't want to be there and would rather be at war than dealing with his crumbling family. Matt, seeing that his father is miserable, gives him the go ahead to return. When Henry is eventually killed by an IED, Matt has a break down struggling to understand why his father would rather die at war than deal with his teenaged son and elderly mother.
- In iCarly, Spencer and Carly's father is rarely seen due to the fact that he's in the Air Force.
- Mega Man Battle Network: Baryl's father was a soldier who fought in foreign countries, so he had to entrust his son to his friend, Dr. Wily. Eventually, Baryl's father died during one of those wars, which sparks Wily's grudge against society and also influences Baryl himself to a degree.
- In Stardew Valley, Sam and Vince's father Kent is absent from town for the first year. He arrives at the beginning of Year 2 and is revealed to be a soldier who spent the last year as a POW in the Gotoro Empire.
- In Archer, Sterling Archer spent much of his childhood being reared by Woodhouse because his mother was still an active black-ops field agent.
- In Avatar: The Last Airbender, Katara's and Sokka's father, Hakoda, has been fighting in the Hundred-Year War for the two years before the start of the story.
- DuckTales (1987): In the first episode, Huey, Dewey, and Louie are sent to live with their great-uncle, Scrooge McDuck, after their former guardian, their uncles Donald Duck, leaves for the navy.
- In a Christmas story of Pocoyo in Season 4, it's shown the Origin Story of the Angry Alien, whose father all Christmas left him alone with his presents to went to the war (implied, in the series is said he went to a "mission"), being Dad the Veteran. That solitude with his toys made him evil and the antagonist he is in the series (in the same episode, he comes back to Pocoyo's world to ruin the Christmas by destroy the presents).