Inspired by /u/kingofthehill5 's post here
I know that initially, Roman legions weren't all that effective against Parthian horsemen. But I also know that Rome was eventually able to conquer [a pretty large chunk of Pathia] (https://en.wikipedia.org/wiki/Mesopotamia_(Roman_province) (for a time) and was decently successful against the Sassanids as well.
What I'm wondering is how did Roman (and/or Persian) tactics and troop compositions change to allow the Romans to have success against Persian horse-archers?
Another, related, question; how about the Mongols? What worked and didn't work against them? I've heard that castles and cities prevented them from invading Europe, but they didn't seem to slow them down in Persia/Russia/China. What, if anything, actually beat them back?
So I wrote a post on why horse archers were so effective a couple of days ago:
As for Roman tactics against them, let's first look at Crassus' campaign. He was a rich man but a horrible commander, and actually was given a good route for his campaign. The King of Armenia offered him safe passage and extra cavalry for his campaign, but Crassus rejected this and instead marched through the desert. One of the most important things to remember when facing an army of horse archers is to avoid terrain that is conducive to their strategies, like the desert, and instead to travel through lands that are bad for cavalry maneuvers, like the Highlands the Armenian king offered passage through. So Crassus chooses the bad option, is led through the desert by Parthian spies without the necessary cavalry support, and ends up getting everyone killed.
One thing to remember about the Romans is that they adapt, and when they suck at something, they hire someone else to do it for them. During Trajan's campaign 150 years later, he brings along a significant portion of auxiliary cavalry. His units stick to highlands and in his case sail along the Euphrates to avoid fighting horse archers in their natural environment. He uses a tactic much like Richard Lionheart's, where you use a natural barrier to protect your rear, line up infantry in front of significant missle fire, and keep your cavalry in reserve to do measured charges to keep chase off the enemy. Even though he is significantly better prepared, they stop at the Iranian plateau as that would have been a dire defeat for the Romans even with Mauretanian horse auxilliaries.
As for the Mongols, they did have problems with castles up until their conquest of China and subsequent absorption of Chinese siege engineers. To say they were stopped by sieges in the west is wrong though. They had the capacity to besiege and take castles, but by the time they reach Hungary they are stretched very thin. They do win multiple battles, but have to withdraw because of the election of a new Great Khan. So why didn't they go back and overrun Europe? As I said before, they were stretched thin and horse archer armies work when they can fight on their terms. The Hungarian plain was the end of the steppes. After the Carpathian Mountains, Europe is heavily forested and has swamps and hill country and mountain ranges. It's just not conducive to the Mongol way of war, and unlike China, is not close enough to make the resources needed worth it.
Source: Erik Hildinger "Warriors of the Steppe"