Even without those chariots the Roman horses and their riders, armored or not, would be dead meat under crossbow fire- the Hun cavalry was!
Napoleanic warfare has proven that a professional and discipline army can advanced under musket and cannon fire in formation and close with the enemy.
There is a flaw to that, I think.
Let's use the typical Roman way of getting around. Marching in columns. Recall the Battle of the Teutoburg Forest? You bet China will try a trick like that. Afterall the Chinese prefered way of warfare is not a frontal assault but through cunning and deceit.
My second example on that - against low morale troops the mass disciplined army may succeed, but there were several situations in the Napoleonic Wars in which the French Column failed to break through the enemy lines, to disastrous effects. As well, this kind of massed infantry creates a lot of unnecessary casualties.
Another disadvantage the Romans has is that the commanders of the legions are usually politicians. Some are just glory-mongers with little or no military experience (of course, everything has its exceptions).
Crossbow troops are likely to be lightly or no armored but they can move away faster. What do you gain from charging them? They will simply fade away and retreat away faster and keep shooting. Weighed less without armor and without need to maintain formation, they will have the advantage of stamina and lightness, and gain the advantage climbing up on higher terrain and rain down more bolts. Chasing down a crossbow unit puts you in a position that you end up in a trap. An infantry formation will be worn down, and will be vulnerable to a cavalry or melee infantry charge. Its mobility, firepower, mobility, firepower, and in the end, the Han crossbow army will dictate the pace, dictate the tempo, dictate the terms, dictate when and where they engage.
What if the Han put those crossbowmen on horses trained to obey by knee pressure when shooting? Not only they would have been more mobile, but also a horse could carry more bolts, and once those are out pikes and long sabers could be used against the Roman cavalry and infantry. The Han would have probably developed field artillery as well, and could have employed Bactrian and/or hybrid camels like the Mongols had done in the 1300s. The Roman cavalry horses would be so scared at the sight of them that their riders would become infantry!Crobato,
These are crossbow men not horse archers. They can't run that fast for that long. Besides they still need to stop to reload (cock the crossbow, especially the ones where you use your feet) and shoot.
The Mongol horsemen carried a composite bow, their favorite, and a long bow. The long bow was used when they were fighting at long range and the composite bow was used in a charge and in fighting from the saddle at close quarters. The composite bow had a pull of 166 pounds and was deadly accurate at a range of 200 to 300 yards. The horse archers carried three quivers each containing different types of arrows for different ranges and uses. One type could penetrate armor, another was used against unprotected troops, and still a third type was used for arrow grenades and flaming naptha. In addition to mounted archers, the army had both light and heavy cavalry. The light cavalry carried bows and javelins, and the heavy cavalry carried lances with hooks on the ends, and sometimes maces. They both carried sabers for hand to hand fighting. Shields were generally used when on guard duty. Eventually their light artillery used various missile-throwing machines, mangonels, catapults, ballista and trebuchets.
Campaigns of the Chinese army to the north and west against the nomads invariably required support by large trains of camels to carry supplies. With the rise of Islam in the seventh century CE, the success of Arab armies in rapidly carving out an empire in the Middle East was due to a considerable degree to their use of camels as cavalry mounts.
Next to elephants these were the tallest and heaviest animals available for cavalry. They are not as agile and slower sprinters than horses. Usage as riding animals, reported from the battle of Qarqar, was more frequent than horses in ancient times. Their advantage was, while they were standing, a mounted archer could aim and shoot with a strong bow from behind an infantry formation. Equipped with small canons, this gave the Afghan troops an advantage during the third battle of Panipat. Another advantage was the shock and awe effect on horses, which had never before smelled these animals. In the battle of Pterium experienced Lydian cavalry suddenly had to struggle with their horses panicking, when trying to face an attack of dromedary riders. The psychological effect of the best trained and most reliable soldiers being overrun in confusion decided the battle.
Crobato,
These are crossbow men not horse archers. They can't run that fast for that long. Besides they still need to stop to reload (cock the crossbow, especially the ones where you use your feet) and shoot.
Roman Legionaires are extremely fit soldiers. Part of their training regimen is to be able to march 30 miles in a day with full armor and baggage (tents, food , etc). Marius mules, rememeber? Their armor is not that heavy as you might think.
Third, crossbow bolts eventually run out of ammo. The Gladius never runs out of ammo. An average musket armed infantry (18th century) carries about 60 bullets, now for crossbow men, I beleive they carry much less due to heavier weight and bulkiness of the bolts. Furthermore, reloads are possible but that would require a large supply chain, which complicates your logistics and slows your mobility.
Fourth, what happens when the Han crossbowmen is in a situation can no longer run, such is a seige (either being seiged or relieving a seige)?
I think you are wrong on your assessment as how one uses a crossbow oriented army. Only horse archers uses a harrass feigned retreat strategy because of their greater mobility. Foot infantry, no matter light cannot run that fast for that long.
An infantry base firepower centric (crossbows) strategy is similar to that of Napoleanic armies. The firepower of the crossbows is used to create gaps and weakpoints of the enemy battle line. Heavy cavalry then charges through this lineto break it. Once the enemy routes, light cavalry (horse archers, etc) pursue to prevent them from rallying.