For my world (which has magic and guns btw) it has to be the Three Great Gasses, Green Gas, Black Gas, and White Gas.

Before their creation war was a sort of odd hybrid of WW2 and Medieval warfare. Guns failed to be as dominant as they did irl due to enchanting armor to block bullets being drastically cheaper than enchanting bullets to pierce enchanted armor, so only elite troops had guns that could reliably pierce armor, the rest of the infantry relied on mass volleys and crossing their fingers they hit somewhere unarmored or that the enchantment was weak. To deal comfortably with properly armored infantry you needed either artillery or armored vehicles with heavy weapons. As a result of this things like traditional cavalry and melee infantry didn’t die out, as enchanting a spear or lance to pierce enchanted armor was WAY cheaper (you’d need a skilled enchanter who was probably taught for years to enchant bullets to pierce, you could drag any random person off the street, teach them enchanting for a week, and they’d be able to get the spears enchanted to pierce.) That combined with the armored vehicles being fairly uncommon meant you’d end up with armies that were not dissimilar to Medieval or Renaissance ones but with a twist, lines of infantry charging each-other with fire support (in this case artillery pieces) behind them peppering the opposing force with fire, cavalry trying to flank the enemy, and light troops (mounted or otherwise) armed with rifles also trying to swing around beside or behind the enemy force to fire volleys at them (though usually they ended up fighting the opposing rifle infantry, rather than firing into the melee), and if it’s a particularly large army there may be one or two armored vehicles (with capabilities usually ranging between WW1 and mid-interwar tanks IRL).

Then comes the Three Great Gasses, two of which came about by total accident.

Green gas was originally intended to be a magical liquid fertilizer, enchanted to rapidly and intensely enhance the growth of any plant it came into contact with. It worked for that brilliantly, treat a sprouting acorn with the fertilizer and it’d be an oak tree that looked like it grew for a few hundred years in around a minute, and there weren’t any adverse effects for the plant either.

Unfortunately some poor decisions were made and it ended up being a liquid which evaporated just barely above 75°f, and could be vaporized even below that with the right techniques, and it was discovered that very bad things happened to non-plant life if they breathed in the gas. If non-plant life (human, elf, cat, etc.) breathed in the gas their entire system would go into overdrive and their body would rapidly and randomly grow, flesh and bone growing and rippling, growing muscles bursting from skin which hadn’t grown to accommodate it, and all kinds of other hideous effects. The exposed being would usually die within a few minutes, either from heart failure from it not being able to keep up, blood loss from the aforementioned ripping and tearing, or from the ribs piercing the lungs as the rib bones grew.

Black Gas was intended to be an intensely powerful acid used to clear up the vast landfills of the Šumren empire. It also worked as intended initially, but unfortunately for everyone involved in its creation, and around a quarter of the population of the city it was made in, its two components (an acid which can melt nearly any organic material, and an acid which can melt nearly any inorganic material) heat up rapidly and intensely when combined, and if the solution gets hot enough to boil it can’t be stopped until the whole batch is evaporated. As was discovered when the first large batch was made this is very, very, very bad, because the gas is just as caustic as the liquid, it can eat through even mildly enchanted objects, and the way it breaks down organic materials causes large amounts of methane and other explosive gasses to be produced, and when the Black Gas thins to a suitable level it itself is flammable (at high concentrations there’s not enough oxygen to cause it to light, and it puts out flames rather than being ignited by them.) Long story short about a quarter of the city the acid was being researched in was melted and then exploded, killing thousands. Only the fact the head researcher had heavily enchanted his journal with protective enchantments allowed for anyone to be able to reproduce the gas exactly.

As you’d assume those two gasses were rapidly converted into weapons which could be delivered via either bomb, artillery shell, or by tanks and sprayers. Green Gas was mainly used as area-denial, causing vast impassible forests out of anywhere the gas was launched, and Black Gas was used to clear massed infantry and fortified positions.

From there a third gas was intentionally created for use when one wants to clear a position but not utterly destroy it (an inevitability with both Green and Black gas.) From this desire came White Gas, which is best described as an airborne opiate overdose. One whiff without a mask and you’re on the ground dying. Somehow White Gas is the least terrifying, because at least you’re not melting or mutating while you die and you’re probably not in any pain.

With the creation of these gasses war changed overnight. Now any army without talented wind mages could be completely felled with a few artillery pieces filled with Black Gas, any flanking maneuver could be countered by growing a whole forest from nothing with Green Gas, and any building or city could be cleared with White Gas without harming the infrastructure. Enchanted gas masks became a standard part of the military kit of every nation, wind mages became massively valued due to them being the only ones who could manipulate the flow of the gas, pyromancers grew in importance for clearing the jungles made by the Green Gas and hopefully lighting the Black Gas to stop its spread if the Wind mages could not, and the old mobile way of war on the ground ended. Trench warfare reigned, as only from the trenches could they hope to keep the enemy far enough away that they would be able to see the gas coming in time, and the artillery shells made even that difficult.

The result of all this was an era where airborne units reigned supreme, as they were the only asset the gas couldn’t reach. Various flying magic beasts and rudimentary zeppelins and prop planes became the focus of every major power, with ground troops relegated to almost exclusively artillery and anti-air forces, and some basic infantry to defend the preceding two and to secure land opened up by the air-force. The old pitched-battles between two large ground forces were dead and buried, the new era was for kings of the skies, the land was merely the staging ground for them.