Historically, this trope might be Newer Than They Think. There is a long literary tradition of glorifying war: bravery, discipline, manliness, martyrdom and the right of the strong to defend the weak, or even to take from the weak. As photographs, film and other forms of mass media from the front became more and more common, the newer trope became more and more mainstream, eventually making said tradition obsolete — it's easier to glorify war when you can't see it up close. And there was one other major contributing factor to the paradigm shift: the increasing brutality of how war was waged. To wit...
Though isolated examples of this date back to The Renaissance or earlier (in non-fiction), the earliest recognized instance of widespread belief in this trope is probably the Thirty Years' War, which ruined the German states and involved such frequent changes of alliances that nobody was really sure why anyone was fighting anybody. The mass armies' constant need to raid the populace for conscripts and food meant that it directly affected a large segment of the population when, before, wars had largely passed the vast majority of the people. As a result, several artists of the period depicted war as a distinctly nasty experience, and popular accounts like sayings seem to confirm a rather gloomy attitude. However, after the Thirty Years' War ended, European militaries grew smaller and wars further from the people (until The Napoleonic Wars, at least), and the trope receded.
The American Civil War prompted another early expression of the trope: US Army General William Sherman is commonly credited with saying "War is Hell." Rebel General Robert E. Lee held a similar sentiment: "It is well that war is so terrible, lest we grow too fond of it." It was the first modern 'Total War' and chewed through a hitherto-unheard-of proportion of that country's countryside, people, and wealth. Furthermore, it was the first war that was extensively photographed and one public exhibition of war-photos, The Dead of Antietam by Mathew Brady, made for such a powerful impression that one reviewer said of it: "Mr. Brady has done something to bring home to us the terrible reality and earnestness of war. If he has not brought bodies and laid them in our dooryards and along the streets, he has done something very like it."
However, since the moral justification for that war was obvious to everyone fighting, in the fields and factories, and overseas, the horrors of war were just that bit more bearable than they might otherwise have been on account of it them being done in a genuinely good cause. The trope was only really... entrenched... in European and Euro-sphere culture during World War I, which was far bloodier and had no great moral cause that the people fighting it could comfort themselves with - though during the war itself the British successfully managed to slander Germany as an aggressive expansionist power that wanted to dominate the world (she'd had problems, but wanting to Take Over the World wasn't one of them). Thanks to near-universal conscription in all the major countries of Europe, a large number of writers, poets and artists of the early 20th century had combat-experience, and they did not like what they saw.
There are several reasons for this. One is that we aren't born as sociopathic soldiers and most modern societies frown on killing for any reason. Most military basic training spends quite a bit of effort to instill into recruits that killing is acceptable and the ends justify the means because their enemies are "savage" and/or "subhuman". For a good look, Full Metal Jacket is a movie to watch. Still, overcoming a lifetime of moral imprinting is very difficult. Many past societies taught their Child Soldiers from birth that killing in war was their noble destiny, firmly establishing that War Is Glorious.
Second, being in constant fear for your life and limb is obviously stressful. Especially in the era of modern industrialized combat, which is more dehumanizing than ancient combat. Back then, if you were a genuine badass with both talent and training, you felt like you were in control of your destiny. Furthermore, war might have actually been fun for those who were actually winning. Modern warfare with its mines, partisans, snipers, machine-guns, grenades, mortars, artillery, gunship-helicopters, ground-attack aircraft, firestorms, poisonous gases, deadly toxins, infectious diseases, killer drones, and massive conventional and nuclear bombs, mean that death can strike without warning or defense - instilling a mindset of paranoia, insignificance, helplessness and nihilistic despair similar to that portrayed by Lovecraftian Fiction. Chemical, biological, and nuclear weapons in particular render your actions more-or-less totally meaningless, as your life itself depends entirely upon whether or when the enemy chooses to use them upon you.
Third, people from societies that promoted war, such as ancient Sparta or Viking-age Scandinavia, thought that they were living in a Crapsack World, with the Rated M for Manly warriors feeling that dying for a good cause was better than waiting to die of old age. In Norse Mythology, those who died bravely in combat went to Valhalla. Even outside the romanticization, many people lived in a world without the benefits of modern science, medicine, or ideas of justice and equity — you spent your life working for an uncaring master, punishments for what we would now consider minor offenses were often brutal and dehumanizing, and people died tragically young from accidents, illnesses, and conditions we would consider preventable often enough for it to be a regular and noticeable occurrence. For many, being a soldier was not necessarily a uniquely brutal or dehumanizing experience compared to the world they were already used to.
And finally, we live in an era of Freedom of Expression where everybody can write and express their opinions. Past societies tended to disdain slaves along with "the common folk" and only recorded the way they lived in general terms. Anything they didn't like could be censored easily. Therefore, anti-war opinions would not be found very often in pre-modern writings, although as early as the 14th century, at least one knight admitted that war could be as terrifying for his own class as it was for the peasant class
; today, however, stories of war being hell get picked up fast.
This doesn't necessarily discredit war or render it obsolete. If anything, this trope has helped promote justifications of conflict along the lines of it being either a necessary evil or an undesirable, last-ditch option when more peaceful means fail. In that regard, especially in North America and Britain, World War II is the shining example of that considering the despicable nature of their enemies and the fact that they were the victors with relatively light losses. In addition, paradoxically and in one of the most confounding ironies known to man, it's been argued that war in some sense has been good for something: namely helping make larger, stable and more peaceful societies possible while reducing the risk of violence over time, and thus less war.
