During the Tang Dynasty, there were only more than 20,000 garrisons in the Anxi Protectorate, which could discipline the Western Regions obediently. But in the Song Dynasty, hundreds of thousands of troops could not suppress the small Western Xia, let alone control the Western Regions.

The Tang Dynasty's garrison of 20,000 troops in the Western Regions was more effective than the hundreds of thousands of garrisons in the northwest of the Song Dynasty.

First, the garrison of the Western Regions of the Tang Dynasty was strong in combat and beat the periphery.

For example, Gao Xianzhi dared to cross the Pamir Plateau with more than 10,000 people, cross the Karakoram Mountains, and defeat the Tibetan elite soldiers even when the soldiers were tired. With this kind of combat effectiveness, I am afraid that the army of the Song Dynasty will not be able to do this job.

Second, the surrounding countries have been beaten by the Tang Dynasty and do not pose a deadly threat.

The main threat to the northern and western regions of the Tang Dynasty was the Turks, who had already split up and the Tang Dynasty defeated the Eastern and Western Turks through two wars. After the Turks were destroyed, those small tribal states in the Western Regions had limited national strength and could not set off any storms at all, as long as a 20,000-strong elite army was stationed, it was enough to deter various vassal states.

In fact, the Han Dynasty was also like this, there were not many garrisons in the Western Regions at all, as long as the most powerful Xiongnu were overturned, other small countries did not have to worry. At that time, Loulan tried to help the Xiongnu deal with the Han Dynasty, but the 700 Han troops destroyed Loulan.

Third, the Tang Dynasty adopted the strategy of razing and dividing the vassal states.

When the Turks were in trouble, the Tang Dynasty generally united with the Uighurs and other tribes against the Turks, and the various tribes checked and balanced each other. More importantly, the Tang Dynasty had a relatively equal ethnic policy towards the vassal states that it submitted, and boldly appointed the talents and troops of the vassal states.

For example, when Su Dingfang defeated the Western Turks, he not only brought the surrendered Turkic tribesmen's cavalry into the battle, but also brought Uighur cavalry. When Gao Xianzhi fought with the Arab Empire, he brought the Qarlu army and the Bahanna army to fight together.

Fourth, the Tang Dynasty garrisoned more than 20,000 troops in the Western Regions, and the support was strong.

The Tang Dynasty set up the Anxi Protectorate and the Northern Court Protectorate in the Western Regions, with 24,000 and 20,000 garrisons respectively, totaling 44,000, and the two capitals supported each other. In addition, the Tang Dynasty occupied the Hexi Corridor, which was the envoy of the Hexi Jiedu and the Longyou Jiedu, and the soldiers and horses of the two towns added up to more than 100,000 and could reinforce the Western Regions at any time.

The situation in the Song Dynasty was very different.

The Song Dynasty garrisoned hundreds of thousands of troops in the northwest, and that was because of the rise of Western Xia, occupying a part of Shaanxi, Qinghai, Gansu and other places. These places are also the areas under the jurisdiction of the Hexi Jiedu and Longyou Jiedu envoys of the Tang Dynasty in the past.

After the rise of Western Xia, its military power was enough to compete with the Song Dynasty, and the Song Dynasty had to gather heavy troops to fight it. Originally, the Western Xia was a small tribe, and its strength was not strong compared to the Song Dynasty, but during the Song Taizong period, a series of wrong strategies were adopted, which led to the gradual rise of the Western Xia.

By the time of Song Zhenzong, Western Xia was already a powerful military force that was difficult to destroy, and Song Zhenzong did not like to fight, so he could only sit back and watch the further rise of Western Xia. When the Tangut people were full-fledged, they couldn't do it without hundreds of thousands of troops there.

Second, the surrounding environment of the Song Dynasty made it necessary for it to be heavily garrisoned on the border.

The Song Dynasty was surrounded by Liao to the north, Western Xia to the northwest, Dali to the southwest, and Tibet to the west.

In this case, the border area must be heavily defended, and the happiest ones are the Liao State and the Western Xia, and many foreign wars in the early Song Dynasty were basically fought with these two. Even if the Song Dynasty conquered Western Xia and recovered the Hexi Corridor, the northwest would have to face the threat of the Uighurs, and the north and northeast would face the threat of the Liao State.

If you want to do it like the Tang Dynasty, there is only one possibility. That is, the Song Dynasty conquered the Western Xia and the Western Regions Uighurs, and then crippled the Liao State and annexed the Dali State in order to relieve the military pressure.

When the Western Regions were pacified, the most powerful tribes were wiped out, and the remaining small tribes could not make waves and did not need too many garrisons, and could use heavy troops to deal with the Liao State in the north and northeast.

But this is impossible, the monarchs and ministers of the Song Dynasty have always had no interest in expanding their territory, and they are more willing to spend money to buy peace.

The Tang army was mainly composed of government soldiers and recruited soldiers, and the Tang Dynasty attached more importance to the army, with a higher status of soldiers and a strong will to fight. The Song army was composed of criminals and refugees who recruited and confiscated the army, and the whole society was permeated with an atmosphere of emphasizing literature over military force, and the status of soldiers was low, and soldiers were often regarded as slaves, so the army's will to fight was extremely low. In this case, the combat effectiveness of the Tang Army and the Song Army is completely incomparable.

To sum up, the Hu and Han regimes of the Tang Dynasty had a huge gap in strength, and with the support of the allies in the periphery and the Western Regions driven by the interests of the Silk Road, the military system was relatively flexible and the status of military generals was high, resulting in the Tang Army's 20,000 troops being able to suppress the Western Regions. However, the gap in the strength of the Hu and Han regimes in the Song Dynasty was not large, and the interests of the Silk Road drove the countries of the Western Regions to form alliances with them, coupled with the national policy of emphasizing literature and suppressing military force, which constrained military generals everywhere, hit enthusiasm, and affected combat efficiency, so the difficulty of expeditions to the Western Regions was much higher, and hundreds of thousands of troops were often required.