I heard a quote once that cavalry is a tactic not a piece of equipment. In the real world, the concept migrated from chariot or horseback to tank to helicopter, and maybe someday it will migrate to robots or something. In your fiction world, the tactic would have had to evolve somehow. I guess that is what your question is, right?

Are military vehicles (tanks, APCS, etc) developed in this alternative time line or were they never invented?

@dunc123 Any technology we have today can probably be assumed to be around as well, unless an answer proposes that continued presence of horseback cavalry would have affected any such development significantly.

I can't see any reason to use horses in a modern conflict except as transport for special forces in extremely rugged terrains (as was the case recently in Afghanistan). Perhaps if the use of mechanized infantry was impossible, for example, if the internal combustion engine (or a substitute) was never invented or developed.

The science-based answer is simply that automatic guns destroy cavalry. AndreiROM's answer covers that very well. They'd need an unscientific upgrade like a stealth field (that doesn't work on vehicles) and even then would only work with long-range shoot-and-run weapons like portable missile launchers.

Are genetically engineered super horses in scope?

@DougT. Sure, I suppose, just make it clear that they're what they are.

If you want to be strictly realistic about it AndreiROM's answer has you covered. Horse cavalry, aside from ceremonial units or special purpose use, is obsolete. They cannot survive in the modern battlefield. Of course you can spin it so the horses are genetically engineered to be faster than APCs, tougher than tanks, and are able to mount railguns or something.

I can envision people controlling drone warfare doing so on horseback. And horses performing the act of launching nuclear weapons. Ceremonially, right?

@NexTerren The to answer covers it extensively... just to make a comment on the error of your approach: you are looking to shoehorn a specific concept into a pre-existing setting, with all the conditions and parameters of that setting still present. That is never going to work. What you need to do is go back and revise the setting itself, and provide a solid, believable foundation as to why horses are still being used. Horses have several great advantages: 1) mobility 2) speed 3) easy to maintain (they don't require fuel... they graze).

@dunc123 or oil very hard to get. Steam-powered vehicles could be useful for hauling heavy loads slowly on flat roads but wouldn't be much use as AFVs

Perhaps you should read Equoid by Charles Stross (if you're prepared to accept super-horses)

Interesting and might be related history.stackexchange.com/questions/230/…

What with urban warfare? Mounted police still exists.

@ChrisH That's not going to help much. Synthetic oil is a thing, and was extensively used in WWII (e.g. the Germans used mechanized infantry, airplanes and tanks extensively, and yet had almost no oil reserves until they captured Bessarabia).

@Luaan for a coal-rich country losing the fuel for its oil-based infrastructure that's one thing. If the oil wasn't there in the first place (globally) a steam/electric/horse powered system would dominate. Synthetic oil from coal isn't exactly the noisy efficient way to use coal.

@ChrisH It's not efficient for power generation, but still useful for vehicles. There's a reason the Germans used synthetic oil instead of coal in their tanks :) I wouldn't be surprised if we returned to synthetic oil from other sources in the future either (after this biofuel fiasco finally dies out :)) - it's quite convenient. Steam is fine for ships and trains, horses are fine for urban travel, but tanks and trucks work quite a lot better even with relatively expensive oil.

@enkryptor Yes, but mounted police are not battlefield cavalry. Apples and oranges comparison.

I know it's nitpicking, but still: the "modern era" began around 1500 AD.

Sorry. I don't think cavalry is a viable option against an army equipped with automatic weapons.

Guns. Gun shoots horse, which is a much bigger target than it's rider and doesn't hide in foxholes, ditches or duck behind fallen logs. Horse falls over, pinning or injuring rider. My \$1 bullet just took out the \$50K you put into the rider and the \$150K you put into the horse. Until you can fix this problem, this is an untenable premise.