The idiom of "East Mountain Rises Again" refers to the East Mountain of Huiji (present-day Shaoxing), which is indeed the name of a mountain, and the main character in the allusion to it is Xie An.
Xie An (320~385), a native of Yangxia, Chen County (present-day Taikang, Henan Province), was born into the Xie Clan, a prestigious family, and his father, elder brother, and younger brother were all officials in the Eastern Jin Dynasty.
However, he was a different kind of person, refusing to serve in the imperial court and living in the eastern mountains of Huiji, where he spent his days traveling with some literati and wandering in nature. His "donkey friends" include Wang Xizhi, Xu Xun and others.
Because of Xie An's great reputation, the court invited him three times to come out of the mountain, and during a short period of time in politics, but soon resigned from the government, the key is to not stand the constraints.
Just as Xie An was reaching middle age, something changed in his family: his father died, and his younger brother, Xie Wan, was made a commoner by the court after he missed his chance to fight the enemy.
The Xie family's power in the court was in tatters, and at this time, the Eastern Jin Dynasty was invaded by a powerful enemy, and the court once again summoned Xie An to come out of the mountain. Xie An accepted the mission regardless of his former enmity, which is the origin of the idiom of "rising again from the east mountain".
In 360 A.D., the former Qin Fu Jian led a million-strong army south to attack Jin, and the court was in an uproar, with some ministers offering to surrender, saying, "Eastern Jin's army, even with all its forces, is not more than 100,000, and there is too great a disparity in strength for this war to be fought.
At that time, the emperor thought of Xie An, and made him a general of conquest. Xie An was 40 years old when he took up the responsibility of saving the country at a time of national crisis.
Xie An first recommended his nephew Xie Xuan to be the vanguard officer, and then he let his nephew lead his family's base of 80,000 men to stop Fu Jian's army, while he sat back and commanded.
His nephew, before leaving, wanted to ask how the battle would be fought, 80,000 versus 1 million, and no one had any idea.
Xie An just said, "The court has its own arrangements, you can rest assured to go. The only thing Xie An could do was to boost morale, and his nephew believed in his uncle's ability, so he wasn't very afraid in his heart.
Xie Xuan led the army and Fu Jian's army in the "Interlake" set up a position, Xie An let the wind and waves, sitting in a fishing boat, he has a number in mind, although there are millions of former Qin, but half of the march on the road, he sent a messenger to convey the order to his nephew, Fu Jian a surprise, to be "attack halfway across the river."
Truthfully marching and fighting in addition to the competition in strategy, a large part of the reason from the timing and geographical location, God blessed the Eastern Jin Dynasty, in such a disparity in the duel, Xie Xuan grasped the first opportunity.
The messenger conveyed Xie An's plan: he ordered Xie Xuan not to wait for Fu Jian's army to gather before going to war, and not to be a foolish and benevolent man like Song Xianggong in the Spring and Autumn Period.
Xie Xuan followed his uncle's plan and suddenly attacked Fu Jian's army halfway across the river. Xie Xuan eventually defeated Fu Jian and won the Battle of the River. This is a typical battle in history, comparable to the "Battle of Red Cliff" in the Three Kingdoms period.
The victory at the Battle of Fu Shui saved the Eastern Jin Dynasty, and Xie An was named above the three princes.
Later, people called such a thing as "rising again" to become an official.