From the Huns through the Mongolians, why were nomadic people from the east able to create such havoc in Europe? Was it an endless 'first mover' advantage with horse domestication? Was it a higher protein diet relative to the more farm-centric Europeans? Why weren't Celts, Ostrogoths, Teutonic knights, or Slavs pushing east? Were Europeans just wasting all their expansive energy on the Crusades?
A few points help in answering your question:
The History has a Selection Bias
The first issue is: Is your question accurate? Keep in mind that we inherited most of our history from the European perspective. There were plenty of cases where Europeans went out and conquered other groups, and the Europeans were just as warlike. The difference is that it largely occurred within Europe. The reason you don't hear about those as much is because Europeans fighting other Europeans, and European armies winning battles was considered typical from a European perspective. With the Mongols and the Huns you have something completely atypical. Large, organized nomadic tribes invading and defeating European armies. These instances were still the exception, not the rule, but now we hear so much more about those events it gives us a biased perspective on history.
Europe was a better target than the Eurasian Steppes
If we assume both groups are militarily capable and warlike then why were the invasions always coming from the Eurasian Steppe into Europe. Two reasons: First, the nomadic horse tribes were more flexible, their armies could adapt to the terrain and distances in Europe, whereas the European armies didn't adapt well to warfare on the Eurasian Steppe (See next section). Also, the Eurasian Steppe was large and not particularly attractive as a territory to conquer. There was definitely wealth in other parts of Asia, but for the Europeans to achieve that they would need to travel long distances and conquer vast amounts of territory that their armies were ill suited to fight on. As Napoleon and Hitler showed, invading Asia from Europe was far more difficult than the other way around.
From the dawn of time until the invention of the railroad, the standard distance an army could move in a single day was about 15 miles. Every army that has managed to improve on that number has met with a lot of success on the battlefield. The Mongols could move 50 miles per day, a number that wouldn't be repeated again until the 20th century. In an era of bulky armies with massive supply trains, the Mongols' mobility was like bringing a gun to a knife fight.
In that sense many of the nomadic tribes shared that advantage, the ability to move fast and in fairly large numbers, so in relation to your question that was a distinct advantage over the Europeans. However, the tribes in Asia were never able to replicate the enduring authority structures you saw in Europe. The Mongols and the Huns united around highly charismatic and successful leaders that came around maybe once every fifty years. It worked at the time, but when those leaders died it inevitably began a breakup of the empires they had forged. The same nomadic traits that gave them an advantage on the battlefield and allowed them to take territory were also a disadvantage when it came to holding that territory.
To answer your question with all that in mind: Their main advantage was speed and flexibility that the European armies could not match. That being said, the Asian tribes were never really able to dominate the Europeans or vice cersa. It was a back and forth and both sides had successes. The successes of the Asians tended to be big and short lived, the successes of the Europeans were smaller but more enduring.
Also, while browsing Quora I came across a similar question. The top answer in this goes into much more detail on the military specifics: http://www.quora.com/Middle-Ages/Did-the-Mongols-have-a-reasonable-chance-of-conquering-Europe-in-the-13th-century-had-Ogedei-not-died-just-before-launching-his-invasion
You might also ask why these people from the steppes also created so much havoc in CHINA. Because they are really two sides of the same coin. In "economic" terms, there are two reasons: 1) "comparative advantage" and 2) "incentives."
To use a model derived from Civilization II (I like to play the Russians and the Mongols on the "real world" map), there are three major items: food, trade, and resources.
By definition, the steppe people are deficient in food and trade, but are comparable to more "settled" and civilized people in resources, and ultimately, weaponry. (At least until the modern age when technology becomes a major factor in weaponry.) "Comparative advantage" suggests that steppe people will specialize in weaponry and war-making, while others with more natural resources will spend more efforts developing agriculture, trade, and, education. Also, the steppe people are more likely to use horses, because the steppe is so poor in food that they have to wander to get enough of it, giving them a further advantage in war.
The other issue is incentive. The impoversished steppe people have every incentive to raid wealthier people for food, resources, and technology. Even if they are better armed, farmers and more settled people aren't likely to uproot their lives for the dubious privilege of chasing steppe people on the plains. Even the Teutonic knights conquered Prussia over one generation, and had they succeeded with the other Baltic states, it would have been one generation at a time. It took Prussia two generations to get three "slices" of Poland. For the Mongols, each "piece" might have represented one YEAR, not one generation, of campaigning.
The danger in the Civilization II game for the Russians and the Mongols is that they will eventually fall behind in technology. That's what happened to the Mongols in "real life." If they can use their military power to stay even in technology and trade (by conquering or coercing more civilized people), while maintaining a military advantage, they can become very powerful, like the Russians.
The period in which this was occurring, roughly 400AD to 1350 (or Andrianople to the popularization of Gunpowder), is what historian Charles Oman referred to as The Age of Cavalry. During this period, there was essentially no good answer to massed Cavalry from the other military arms.
The problem Europe (and China, and other settled farming societies) had here was that a good cavalry arm of the day required an immense amount of training. Pastoral societies (which the Eurasian steppe was uniquely suited to) essentially get this for free, as every adult male practically spends their life on horseback just to perform the herding functions they need to survive. At need, every adult male in this society can be drafted into the military as expert horsemen. However, a peasant farming society cannot do this, so militarily they just can't compete.
Different societies dealt with this problem in different ways. China was rich enough to buy off the pastoralists a lot of the time, and coherent enough to absorb their conquerors when they couldn't buy them off. The Byzantines (nee Eastern Roman Empire), were sitting on a rich trading route, and used a combination of buying off the pastoralists, and very strong fortifications. The poor western empire disintegrated, but eventually hit upon a kind of caste society where the lower classes, along with all remaining governmental machinery, worked entirely to support an upper class of horsemen.
Here is an outline of a few sentences distilled from many long paragraphs at my previously mentioned website.
The apparent military superiority of the horse-mounted nomads of central Eurasia during ancient and medieval times was due to:
For Mongols specifically, it was in part their unparallelled-till-20th-century tactical flexibility.
At times, all of these people pushed eastwards. It just depended on relative technology and strength of population.
Celts: At maximum expansion, about 270 BC, they invaded Poland, the Balkans, and even central Anatolia and the Ukraine.
Goths: Before moving west ahead of the Huns, the Germanic Goths moved from Sweden and the Baltic to the steppes around the Black Sea by around 100 AD.
Slavs: During the expansion of the Slavs about 600 AD they expanded deep into Russia, eastward.
Teutonic Knights: An odd choice in that the group is best known for its Eastward Conquests of the Baltic.
I think the answer is demography. In European urban and agricultural societies the main limiting factor was the extent of available land. While Europeans had high fertility rate at the time, most children could not reach their puberty due to food shortages.
At the same time, the nomad peoples could just divide and find new pastures. And the territory of the Asian steppes is enormous.
In most cases where Mongols were faced with numerically equal enemy, they retreated. There are very little cases where Mongols successfully defeated their enemies with equal number of troops. And in most battles even when they were victorious, they suffered greater losses.
As to the question why Europeans did not expand to the steppes in the East, this is simple: there was nothing to grab in the steppes. No treasures, no cities, no profit. And the land was poor for settled agriculture. On the other side, Europeans took considerable and repeated efforts to expand to the Middle East because that region had things to take.
Communications/Logistics. Eastern Europe was pretty undeveloped with poor local infrastructure to support armies or any great wealth to attract conquerers. Pushing out from Europe across the steppes just isn't attractive and is difficult. The mongols or other horse nomads are able to cross the area more easily, moving quicker and able too graze their horses.
I dont agree that asian steppe armies "dominated". Difficult armies to fight but steppe armies never made that big a inroads, the Mongols nibbled the edges, defeating two of the smaller kingdoms in a couple of battles, the Huns get a way in but only controlled a small area of civilised Europe at the time for a short period.
Horse nomad armies are hard to 'force' to fight not having a logistical base to defend, generally can pick and chose win to fight being much more strategically mobile generally. (though they need large grasslands to support their horses, and it takes lot of hours out of the day to graze horses, so they might be very quick in the short term, but slower over a longer term) So generally they withdraw rather that be defeated.
Horse Nomad armies are a different military paradigm from civilised armies and generally the civilised armies are poorly adapted for fighting them. They can 'appear' quickly, moving quickly causing a lot of confusion, as armies used to vastly different opponents fail to adjust. A couple of battles in the crusades the turkish horse archer armies were smashed by european Knights failing to adjust themselves, it's not certainly that the nomad horse archer army just wins. The Mongols had superior élan, discipline and organisation compared to your average horse nomad army.
The Saxons at one point in the 8-9th centuries settled in eastern Poland, Western Belarus. After an invasion of Bulgars and Slavs in the 10th century a sundering occurred in which a group of Saxons migrated to the lower Volga region. This tribe of Germanic east Saxons formed a small empire and expanded further east into the Kirghiz regions and possible as far as Manchuria. This is as far as I know the only European tribe that reached Far East Asia. Albeit much assimilation and inter-mixing occurred along the way. They also apparently adopted Islam at some point.
Your original question does indeed provoke much intrigue. There are a billion reasons. When the Huns invaded, Rome was already in a decline from internal issues and Germanic invasions. If the Huns invaded 3 centuries earlier the Romans would have handled them. During the Mongol invasions Poland and Russia were fragmented, although I'm not sure why the Hungarians and Bulgarians were defeated so easily as they had established kingdoms and used the nearly the same tactics as the Mongols. conceptual information on the East Saxins John Chambers, The Devil's Horsemen: The Mongol Invasion of Europe, Atheneum, 1979. p. 31, mentions these saxons.
Focusing on the very sentence of your question, one "simple" answer to give regarding the superiority of the steppe riders was the use of the humble stirrup.
Their recorded use of the stirrup to stabilise the rider in the saddle, allowing them to twist and lean, gave them a huge mobility advantage.
In "the West" the stirrup was just a mounting aid (the word even etymologically means that, just as, for example, the German term Steigbuegel).
So, you ended up with major armies that were able to fire and whirl with unprecedented speed, while their opponents could mostly only barrel forward in a fairly straight line. Any sudden turn and you had to hold on somehow. Without stirrups, using only your thighs to brace is haphazard at best and will tire you out in no time at the worst. Meaning you had to hold on with your hands, while your opponents used those hands to fire volleys of arrows non-stop and ride circles around you. Also, the only thing you had to hold would likely be your reins, a sure way to cut the speed of your horse. Which was precisely not what you likely had in mind.
Contrary to some notions on the stirrup bringing about the knight with shield and lance who would brace themselves in the stirrup in their charge, the main advantage was indeed for the mounted archery used by the steppe people to devastating effect. As Mounted Archers of the Steppe 600BC-AD1300 by Antony Karasular explains, "The stirrup, though often touted as a big step in seat security for sword-, spear- and lance-wielding horsemen, really had more to do with creating a a stable base for mounted archery. It is thus not surprising that it is first seen among mounted archers." (page 47).
As this source also states, solid evidence of (bilateral) stirrups comes from the 1st century AD. The also coincides with the prerequisite invention of the saddle tree, creating a solid, stable frame on the horse's back which locks it in place while distributing weight. For lance warfare, the stirrup is possibly of less importance (ibid.), but as stated above, the steppe peoples were renowned (notorious?) for their use of hit&run archery attacks, not the armoured attacks of medieval knights.
I am still trying to get my hands on the full proceedings of "Stirrups and archaeological populations: Bio-anthropological considerations for determining their use based on the skeletons of two Steppe riders" Summary here, which should give some fascinating additional information on how the physique of these people changed.
The ability of steppe riders and mounted archers to fire with their bodies twisted around while riding away from the enemy has even made it into the English language: the "parting shot" as a snide remark made in passing or left while walking away from an argument has its roots in the Parthian Shot, which describes this technique and according to Livy in "The Life of Crassus" was the main contributor to one of Rome's worst defeats in 53BC at Carrhae (not the renowned Parthian cataphracts, i.e. armoured "knights").
Page 53 of "Mounted Archers" provides a great summary of how all the previous items combined to offer an answer to the original question and, while fascinating, is too long to quote here in full.
Speaking as a rider, I can tell you first hand that riding without stirrups, while a good balance exercise, is not something you want to do when the going gets a bit rough and the speed gets high. A flood of YouTube videos showing people jumping without saddles, stirrups, bridles notwithstanding.
You will find discussions of this aplenty in equestrian literature. Interestingly enough, one of the most recent mentions of it was on one of the earlier episode of the History of English podcast, in the episode of Gothic and other Germanic migrations under pressure of the Huns.
Excellent question! I respectfully invite you to visit my new website "The Nomadic Horse Peoples of Central Asia" at http://www.horsenomads.info Please note it is dot-info, not dot-com. I think the textual "Introduction" section of my website addresses your question rather well. Thanks! Stephen W. Richey