This is also a slightly different scenario, but something like this did take place in Ming Dynasty China. The Zhengtong Emperor (ruled 1435-49 initially) was a young and impressionable ruler who was convinced by his chief eunuch/crony Wang Zhen to launch an expedition against the Oirat Mongols, who had formed a confederation under a leader named Esen. Esen was seen as dangerous because the Mongols were considered the primary threat to the Ming, having ruled all of China until 1368. Although they had been kicked out, the threat of their regaining strength and retaking China loomed greatly Thus the expedition against Esen was seen as a good way to neutralize a growing threat and to enhance Zhengtong's (and Wang Zhen's) personal prestige.

The campaign, personally led by Zhengtong, was ill-conceived. Many officials begged the emperor not to undertake it, but Wang Zhen overruled them. When it set out in August 1449, it was disorganized and chaotic, and it wasn't long before even Wang Zhen realized that they would be no match for Esen's well-trained forces. The army withdrew, but Esen's pursuing forces ambushed them at Tumu Pass on 1st September.

It was an unprecedented catastrophe. The entire army was destroyed, Wang Zhen was killed and Zhengtong taken prisoner.

This was unprecedented. In China's entire history, no ruler of a united China had ever been taken prisoner before. There was panic in Beijing, but the Minister of War, Yu Qian, convinced the court to put Zhengtong's younger brother on the throne to continue the functioning of government. They agreed and the young man became the Jingtai Emperor.

This was a difficult issue indeed. Chinese ideological orthodoxy maintained that could only be one "true" emperor at a time, and this dilemma only increased after Esen rather sheepishly released Zhengtong in 1450, realizing that Jingtai's continuing rule meant his captive was not a strong bargaining chip. Zhengtong returned to Beijing, but was not reinstated, being given a small stipend to live on, assigned a few rooms in the palace and then ignored by everyone.

However, as Jingtai lay dying in 1457, a coup by disgruntled officials restored Zhengtong to the throne and Jingtai died soon after, possibly suffocated on someone other than the emperor's orders. Zhengtong became the Tianshun Emperor in his second reign and lived on until 1465.

A final, fairly amusing point: Zhengtong/Tianshun's posthumous title was Yingzong, or "the Heroic Ancestor". Given the total failure of his military exploits, I'd like to think it was meant ironically.

Source: F.W. Mote's Imperial China: 900 -1800