There is a good chinese article on Tang military and their tactics at
http://bbs.hefei.cc/archiver/?tid-382544.htmlI'll summarize the main points:
Basically, during the rebllion against the late Sui dynasty, the Tang rebels were able to make full use of their high morale and mobility in tactics to beat the heavily but cumbersome equipped Sui army. It destroyed the reliance on heavy cavalry that was present from Age of fragmentation till Sui dynasty.
During war, the outcome of "light cavalry vs heavy cavalry" fighting was based on the comparison between mobility and the armour protection capability. But finally, it was the resurgence of the light cavalry that became the main power behind Tang's army. But heavy cavalry continued to exist, just that its importance has been less emphasized than the light cavalry.
In the Tang army, the number of cavalry troops are alot. The ratio between infantry and cavalry is about 3:1. This was something that other dynasty were not able to achieve. Tang dynasty was the peak era of cavalry in China, and it was also the zenith of chinese power.
Ever since Qin and Han dynasty, the Tang produced the most horses. During the time when Li Yuan rebelled against Sui dynasty in Taiyuan, the Tang were able to obtain 2000 horse's assistance from the Turks (Tujue). It also obtained about 3000 Sui horses from Longyou. This was the earliest base for development of Tang's light cavalry. From then on, the practice of rearing horses started to become popular. The Tang also learnt from past experience and emphasize the technique and method on rearing horese. They also bought horses from the northern nomads, in order to intermingle their horses (letting them mate) in order to produce horses which are strong and healthy. The number of horses produced also greatly exceeded that of past dynasty. From the times of Taizong, there were 750,600 horses. Whenever the Tang launched any military expedition, it would choose good horses that were strong and healthy. Because of rearing good quality horses, the Tang army's horses were of equal quality as compared to the Turk's horses in the western region. In comparison to Tang, the han dynasty horses were alot but their qualities were not good. After the reign of Tianbao, every troops had up to 10,000 horses.
The "policy of horses" is the basis for building up a strong cavalry army. Thus, the Tang army cavalry forces were stronger than that of any past dynasties.
The Tang army were able to fight several victorious battle due to the following reasons:
1. After Jin dynasty, the central plain of China were in turmoil. The northern nomads invaded China and there were countless of wars. In order to protect themselves, the citizens build several walls and fortress and trained themselves in the art of fighting (using swords or archers), thus it produced an atmosphere that favoured fighting for the military. Several aristocrat and officials also had their own private army, these army were actually quite alot, and when combined together, they can form the whole army of a country. They were also well-equipped (consisted of mainly heavily-armoured heavy cavalry). Because of countless years of warfare, these soldiers were skilled and experienced in fighting. In addition, because of great mixing between the northern nomads with the han-chinese, they caused the han-chinese to be more ferocious and more war-like. Before Tang, China inherited a culture of war-like and atmosphere of fighting.
During the early yars of border wars in Tang dynasty, esp. during Taizong's period, it practised a strategy of what's called "using the foreigners to fight against foreigners". In several battles, it has the support or logistic backup from the northern nomads of the north-west. In addition, the Tang army were skilled in fighting and thus won several battles. The winning of battles also mean that the Tang obtained several horses, and thus the quantity of horses kept increasing during Tang. It wasn't until the Rebellion of Anshi that the link betwen the west was cut off and the Turks once again rebelled, and the "policy of horses" began to decline. Its Fubing military system also began to be damage, and several of its conscript were not abel to fight. Thus it causes the decline of the Tang army by the late Tang period.
Tang Military Organisation
The Tang military organisation is known as "Fubing system 府兵制" (The system of House Military). The "Fu 府" can mean an official 'house' and were distributed across the country. It was also known as "Wei Fu 卫府" (Protection House). In the northern frontier, there were guards troops. The Fu troops were regularly dispatched on rotation to guard the frontier. The Fu soldiers were permanent soldiers for life. They had to service the army regularly. After his service period was over, he can return home. During his service in the army, he must bring his own weapon, food, horse and clothings etc. Because of the popular practice of rearing horses among the public, the army did not need to fear a lack of horse. Even for infantry, they were also equipped with large number of horses, thus the Tang army's marches and traveling speed was quite fast.
The Tang troops were organised into "Dui 队" (team) as the most basic tactical unit. One "Dui 队" comprises of 50 men organised into of 5 "Huo 火". Each "Huo" has 6 horses (infantry). Above Dui, it is a "Tuan 团" (battalion), and above "Tuan", it is a "Jun 军" (army). Whenever a general went on expedition, it would have about 20,000 troops, which were divided 7 "Jun" (comprises of front, back, left, right, central army etc.). Within these 20,000 troops, there were 14,000 different combat units and 6000 supplies/logistics units. Out of these 14,000 combat units, it would consist of 10,000 infantry, 4,000 cavalry, each distributed across 7 "Jun".
The central army (jun) is the largest while the other 6 armies would be close to one another. Within the infantry, there would 2,000 crossbowers, 2,200 archers, 2,900 raiding troops (commando) and 2,900 special troops. From the ratio of various units, we can see that the calvary units are the largest and became the main force of Tang army. With such a strong cavalry, the tactic combat capability of Tang army is undeniably strong.
The Tang cavalry were well-equipped. For the short-sword of cavalry, all uses Dao or sword. For long weapons, they uses horse-lances (shuo 槊) as well as axes. For long-distance strike, they uses cross-bows. Li Shimin was capable of shooting from bows. General Weichi Jinde was capable of using lance.
In terms of cavalry tactics of Tang , what's most amazing was the sudden raid and force raiding tactic of its light cavalry. In most circumstances, the cavalry combined its main force to carry out long-distance flanking. Li Shimin and General Lijing were especially skilful in the use of sudden raid using cavalry. For instance, Li Shimin led cavalry to raid Xue Rengao, Li Jing used 3,000 cavalry to raid Dingxiang etc. These kind of tactic make use of 'surprise attack' and was copied from Tang till 5 dynasties period.
Also, in terms of cavalry tactic, the Tang also greatly emphasized the smooth combination between infantry and cavalry, and make full use of their strength to increase combat capability. For instnace, during the rebellion of Anshi when capturing Chang'an and Luoyang cities, Guo Ziyi made use of infantry advance in the front, while using the Uighur cavalry to attack from the back to achieve success.