Liu Yu's expeditions

Liu Yu's expeditions were a series of successful campaigns mounted by Jin China from 409 CE to 416 CE against Southern Yan, Later Qin, Northern Wei and Xia that successfully recovered all of China's territory south of the Yellow River with the exception of the Chang'an area(which was taken by Xia). This victories were the basis of the prosperity of the Reign of Yuanjia

Background
Following Jin's victory at the Battle of Fei River, North China divided into several non-ethnically Chinese lead states. After Liu Yu, one of the most excellent generals of the Northern and Southern dynasties period, came to power in Jin, Liu Yu saw the opportunity to recover the areas of China's heartland which were under foreign rule.

1st expedition against Southern Yan
Liu Yu's first expedition was against Southern Yan in 409 CE. The Jin army attacked from Nanjing to Xiapei, and then to Longchen. Along the roads, Jin's forces built fortresses to prevent Southern Yan forces from cutting off their supplies. By May, Jin forces reached the Southern Yan capital at Guangdu. When he reached the area, Liu Yu saw that the ground was covered with grain and he said to his men that: "The Southern Yan ruler is already in my hand", as there would be no supply problems. In February 410 CE, Jin forces captured Guangdu and put an end to Southern Yan.

2nd expedition against Later Qin
In 416 CE, Liu Yu launched a major attack against Later Qin, defeating its general, Yao Shao. After this victory, Liu Yu recovered Luoyang, the former capital of the Jin Dynasty and was crowned Duke of Song. To complete his victory, he sent two armies into Shaanxi to engage the remnants of Later Qin's forces.

On their way there, the Jin forces were harassed by Northern Wei cavalry, who attacked the Jin forces' supply routes. In a major engagement, Jin forces defeated Northern Wei forces using crossbows to launch spears through the Wei forces, frightening them and reputedly killing some 30,000 Wei soldiers. Following this victory, Jin forces continued on Later Qin and captured its capital Chang'an, destroying it.

Loss of Chang'an region
Following this victory, it seemed that Jin would quickly destroy Xia, Northern Wei and the remaining barbarian states, reunifying China. However, Liu Yu went back to Nanjing on rumors of the death of Emperor An of Jin, leaving his general Wang Zhen'en in charge. Xia forces took the opportunity to attack, and captured the Chang'an region from Jin. Nevertheless, Jin still held most of China's territory south of the Yellow river, the traditional heartland of China.

Aftermath
These victories laid the foundation for the Reign of Yuanjia, a period of relative prosperity as the Chinese heartlands fell under native rule rather than be subject to threats of invasion or constant warfare.