An even more recent example is Gundam SEED which slides to the far end of the Cynical side of the scale. War becomes humanity's excuse to perpetrate human experimentation, nuclear holocaust, and mass betrayal. Given that the war was started in the first place to commit genocide against a subspecies of humanity, and it eventually evolves to the point where both sides refuse to accept each others existence and commit ruthless atrocities in an attempt to end all of mankind, yes, war is indeed HELL.
To quote Kamille Bidan (Z Gundam) in Dynasty Warriors Gundam 2: "Everyone's... dying. It wasn't supposed to be like this... Are you happy now? Are you satisfied? So. Who's gonna celebrate now, huh?" (He's actually talking to both Quattro, Scirocco and Haman at this point, as they're the respective leaders of each their factions. Yes, at this point he's willing to question his own commanding officer's motives.)
The One Year War. It's the most famous war in the Gundam franchise, with half a dozen or more side-stories elaborating on it. In fact, in the course of the war, the Zeons are willing to drop a giant space colony, housing 3 million civilians, down on earth, killing the civilians in the colony with gas beforehand, and destroying quite a big portion of Australia, create giant machines that can kill thousands of people in seconds, and blame it all on the Earth Federation. In fact, they start the war by dropping a colony on Sydney, leaving a crater that can be seen clearly when Kou comes to Sydney in 0083, four years after the war. It doesn't get much better that most main characters in 0079-series are mentally scarred, or break down.
Even Mobile Suit Gundam ZZ. one of the goofiest, most light-hearted shows in the Universal Century, chronicles the slow descent into hell of a group of happy-go-lucky kids caught on the periphery of the conflict. By the end of the show, the survivors do manage to go on with their lives, but they ain't so happy-go-lucky no more. A couple even head off to the Jupiter colonies, in an attempt to get as far away from Earth and its wars as humanly possible.
Mobile Suit Gundam F91 deserves a mention, as when the colony is attacked by the Crossbone Vanguard, more than hundreds of people are killed at the beginning of the invasion.
In the final arc of Gundam Wing, Zechs and Trieze take command of the militaries of the colonies and the Earth respectively and launch a huge bloody war just to finally convince humanity that War Is Hell. Later works in the same setting show that the lesson didn't stick.
Also in Gundam Wing, an early episode has Wufei planting bombs in the trainee dormitories of a base. Said trainees were going to ship out tomorrow and had literally just graduated. We get to hear them all scream as the bombs explode, too.
The Ishval Massacre in Fullmetal Alchemist is wall to wall bodies; the Ishvalans are either being massacred or are killing the Amestrians in a last ditch effort to survive. By the end of the flashback everyone is left traumatized, and most of the rest are Ax-Crazy to begin with.
Mazinger Z: From the opening narration from New Mazinger (an one-volume-long alternate manga version published in 1988): "A. D. 220X... Hostilities between north and south grow in fury as savage combat with new and ever more destructive weapons lays waste the once pastoral Earth. The remnants of mankind burrow deep beneath the surface. Their citadels, screened against the deadly bath of radiation, poke through the polluted soil like foul, mutant flowers. Their warriors, encased in giant combat armor against the air that once gave man life, live only to fight, and with luck, to fight again. Today, as every day, the flames of war rage in every corner of the globe. War without quarter. War without end. War for a race that has forgotten all other ways of life."
This is, actually, one of core messages of UFO Robo Grendizer, and it is delivered in a pretty heavy-handed way. The Vegans tried to take over Fleed to settle on it but their own weapons turned the planet into an uninhabitable, radioactive chunk of rock (and the scenes of the Fleedian genocide were especially Nightmare Fuel). The only thing their expansionist war achieved was the destruction of several planets and their own annihilation. The war scenes were depicted in a very harsh fashion. And several characters argued nothing could be gained from that conflict:
Hikaru: “Why friends must fight each other? It’s all the same. It doesn’t matter who wins… There’s no way peace can result from this fight!”
Rubina: "If this war continues, not only all the Vegans but Earth itself will be destroyed."
Naruto. In fact, Madara said "For a child, war is hell". War was responsible for making Itachi who he is and also very much had something to do with Pain's motives. When the three Sannin met Nagato (Pain), Yahiko and Konan, Orochimaru suggested killing them because only more pain and hell would await them and tragically that turns out to be the case.
The Fourth Shinobi World War defines this trope when you hear the death toll of the Alliance forces for the first day. 40,000 Ninja and samurai died in a single day of fighting. But due to A Million Is a Statistic, most people don't care unless a named character dies.
As of chapter 615, Shikaku, Inoichi, Ao, Mabui, the entire Kumogakure village, and Neji have all died in the War against Obito and Madara.
Flashbacks show that the era constant warfare that Hashirama and Madara grew up in was even worse than anything in the present day. The average life expectancy for soldiers and civilians alike was 30 thanks to the deaths of so many child soldiers. Illustrated perfectly in one page depicting Hashirama's youngest brother Itama crying in fear while surrounded by a band of angry Uchiha, with the next panel showing his bloody corpse.
Zambot3 explored this trope. Child Soldiers forced to handle weapons and fight a faceless enemy? Check. People dying suffering and dying the whole time? Check. People turned into human bombs? Check. Cities being destroyed? Check. Both sides battling among the ruins of cities already destroyed in previous battles? Check. Every side thinking the other side are the evil ones? Check. It is no wonder this anime was done by the creator of Gundam during one of his Kill 'em All phases.
And then Tomino took it up to eleven when he created Ideon. A war between two sides starts cause a misunderstanding. The result? Tons of deaths, destruction, suffering, mindscrew and the destruction of the universe.
In Nausicaä of the Valley of the Wind, most if not all the war scenes are drawn in explicit and gritty detail, often squeezed into tiny panels making them look as claustrophobic and uncomfortable as possible. Well, this IS a Miyazaki work...
Vinland Saga delivers this message with surprising subtlety.
Now and Then, Here and There dumps an innocent young protagonist into a world made hell by war. In this world children are the targets of atrocities committed by other children. Neighbouring villages are raided for vital supplies and young boys to be conscripted into the insane king Hamdo's army. Women and young girls are captured to be passed around to and raped by Hellywood soldiers as a reward for good performance in the hope that they will become pregnant and provide future soldiers and breeders.
Jin-Roh: The Wolf Brigade paints a rather vivid image of a post World War Two Japan where the Nazis were victorious, with rioting in the streets, child terrorists and the realpolitica power plays with the CAPO and Public Security backstabbing each other. Bonus points for the main character shooting his girlfriend rebel before a sniper killed both of them.
The Front Mission expanded media, such as the Dog Life & Dog Style manga, are very much in the War Is Hell territory. In Dog Life & Dog Style, the brutality of war in general is shown through as Kenichi Inuzuka, a Japanese journalist who winds up in the wrong place and wrong time to witness some of the worst atrocities in the midst of the 2nd Huffman Conflict and those who are affected by it. He brings out the truth of war to the world amidst censorship and to some of the people whom he interviews. The seinen manga goes from a violent form of war is hell to a psychological form. In the first volume alone, Inuzuka casually takes pictures of about every single atrocity of war, from open battlefield rape of some random girl to the amount of skulls hauled by a wanzer pilot to mark his kills. In the second volume, a particularly tragic story shows the exploits of O.C.U. Captain Ren Akagi who supposedly saves a young woman, Kino Margo Seletskaya, from rape and performs a Heroic Sacrifice to get her out of the warzone. It is then revealed that Kino was a prostitute who wasn't so much as raped as performing her trade on the field and that it wasn't the first time that happened. Thankfully, Ren survives but he does not walk out unscathed, losing an arm and two eyes as well as learning the unpleasant truth about her trade.
Barefoot Gen, anyone? This one portrays the bombing of Hiroshima and its aftermath, as experienced by a six-year-old boy and his family.
The core tenet of the Third Squad of Shinigami in Bleach is that battles are something to be dreaded. They are a horrible, terrifying experience that should not be glorified. This is key to ensure that those who participate become so fearful of battle that they would rather find peace than seek war.
While the Monster Rancher anime is a childrens' series, lots of monsters, good and bad, die in it, and the characters are shown that a past war nearly wiped out civilization as they knew it. When friendly characters die, it's often very upsetting, even traumatizing Genki on some occasions. And let's not get into the second season's Downer Ending...
Legend of Galactic Heroes emphasised this trope in various ways, from the protagonist's anguishing over the deaths of countless soldiers under his command right down to particularly graphic scenes of destruction that both warring factions experience.
This is the chief theme of the Area 88 manga and OVA. More generally, the series is about protagonist Kazama Shin's journey through hell after his best friend tricks him into joining a mercenary air force.
Shin tells the base's resident Arms Dealer, McCoy, that he'll go to hell for selling weapons. McCoy replies that he's already there.
The War Of The Best arc in One Piece, which dealt with The Marines fighting Whitebeard, plays this trope straight. Both sides start off fresh and eager for battle. Near the end, almost everyone is splattered with blood, marines and pirates alike lay dead, or injured on the ground, trampled by allies and enemies alike who don't care. Soldiers who try to desert get killed mid-battle by Akainu, and the people who decide to help their fallen comrades? Get shot as well, with a massive body count for both sides in the end. Two major characters end up dying, including the protagonist's brother. Not to mention that the end result made everything -even worse- in the long run.
Persistantly present in Attack on Titan, especially in scenes before and after various battles where soldiers are seen to break down in terror or struggle to not flee.
The Aincrad Arc of Sword Art Online. It all started the day the game launched, when the players were stunned to find themselves trapped in the game, and dying in the game means dying for real. In the ensuing arc, most episodes have characters die in the episode, and most of the ones that don't involve murder investigations. Some of the players snapped and committed suicide; others grew so nihilistic that they committed murder for sport. Back home, nobody other than the veterans wants to remember that the war even happened.