What caused the war to break out when the Tang Dynasty destroyed Korea?

The battle to destroy Koguryo in the Tang Dynasty was not that big countries bullied small countries, but that Koguryo's powerful minister Gai Suwen monopolized state affairs, sent troops to attack Silla, and prevented Silla from paying tribute to the Tang Dynasty, and the Tang Dynasty sent troops to safeguard its own interests.

Goguryeo, Baekje and Silla have always been attached to the Han Dynasty.

In AD 3, Koguryo moved the capital to the present territory of Ji 'an City, Jilin Province, where the political center has been for more than 400 years. After the Eastern Han Dynasty became strong, it developed in Wei, Jin, Southern and Northern Dynasties. After 427 AD, it was attacked by the Han Dynasty and moved to Pyongyang. However, as a local political power in China, its independence is getting stronger and stronger, and it has long competed with Baekje and Silla for territory. After the destruction of the Turks in the Tang Dynasty, Goguryeo, Silla and Baekje all surrendered to the Tang Dynasty.

Koguryo changed the dynasty and attacked Silla, cutting off the passage from Silla to the Tang Dynasty.

In 643 AD, Silla, as a vassal state of the Tang Dynasty, sent people to the Tang Dynasty to complain that Goguryeo and Baekje had invaded its territory. Sui Tang mediated, but inherited the Koguryo king Liu Rong, who was in friendship with the Tang Dynasty at that time. After being killed by Su Wen, a powerful minister, he took charge of state affairs and refused to listen to Tang Dynasty's persuasion. After capturing Silla, he cut off the passage between Silla and Datang. And sent people to contact Xue Yantuo Khan who defected from the Turks and the Tang Dynasty, preparing to challenge the Tang Dynasty. The unbearable Emperor Taizong appointed Sean and Li Ji as generals, and led his own army to personally levy Koguryo.

Challenge the bottom line of Datang and be destroyed.

From the beginning of the invasion, Li Daozong attacked a new city, Li Shiji attacked a city, and Sean attacked Beishacheng, all of which showed the strength of the Tang Dynasty. Although Anshi City was not broken when it met with stubborn resistance, Emperor Taizong's team felt pity for the soldiers because of the weather, and returned to North Korea, but also retreated calmly at the gate. In 647 AD, Emperor Taizong ordered Niu Jinda and Li Ji to lead troops into Koguryo by land and water. The two men fought more than a hundred times, and Tang Jun won every battle. Baekje took the opportunity to occupy a dozen cities. At the end of 660 AD, the Tang Dynasty sent Qi, Liu and other soldiers to attack Koguryo. Exhausted Goguryeo turned to Japan and Baekje, but to no avail. Goguryeo finally died after more than a month of troops stationed in Pyongyang.

The demise of Koguryo was an important war for the Tang Dynasty to show its strength to the outside world. During the Tang Dynasty, Silla's territory was still south of Datong River and Pyongyang. ?