Long ago, two groups fought a war. 
Long ago, two lands fought a war. 
At first, Goguryeo was very strong. They fought off the Tang armies. They kept their land safe for a while.
Then, things began to change. A leader in Goguryeo died. This caused big fights among his family. People became sad and tired.
Because of this, the Tang and Silla lands joined together. They attacked from two sides. It was hard for Goguryeo to fight both.
In the end, the war was over. The lands changed forever. It was a very big moment in history.
Long ago, a big war happened in Korea. 
At first, Goguryeo was very strong. They fought off many Tang armies. One famous place was Ansi Fortress. The people there defended the walls well. They even stopped a huge mound from being built. The Tang army had to leave because of the cold weather.
Later, things changed. The Tang joined with a land called Silla. Together, they first conquered Baekje in 660. Then, they turned their focus to Goguryeo.
A leader named Yŏn Kaesomun died in 666. This caused big fights within his own family. His sons even joined the enemy sides. This made Goguryeo very weak.
In 668, the Tang and Silla armies attacked again. They took the city of Pyongyang. This war ended the Three Kingdoms period. It changed how people lived in that part of the world forever.
The Goguryeo–Tang War was a long series of conflicts in East Asia. It took place between 645 and 668. The war was fought between the Goguryeo kingdom and the Tang dynasty. 
The fighting often involved large armies and many different strategies. One way the war was fought was through long sieges. A siege is when an army surrounds a city to force it to surrender. 
The war began because of political changes and old disagreements. In 642, a leader named Yŏn Kaesomun took control of Goguryeo. He killed the previous king and took the throne. Emperor Taizong of the Tang dynasty used this event as a reason to invade. 
Many specific battles and dates defined this era. In 645, the Tang army captured the city of Gaimou. They took 100,000 shi of grain from that location. Later, the defenders at Ansi Fortress held off the Tang for many weeks. In 660, the Tang and Silla alliance successfully conquered Baekje. 
This war is important because it reshaped how people lived together. After the war, the Tang dynasty tried to rule the new lands. They set up special offices to manage the former territories. 
The Goguryeo–Tang War was a major series of military conflicts in East Asia. It lasted from 645 to 668. The war was fought primarily between the Goguryeo kingdom and the Tang dynasty. 
Political instability in the region triggered the fighting. In 642, a military dictator named Yŏn Kaesomun seized power in Goguryeo. He murdered over 180 aristocrats and killed the king. He then placed a puppet king on the throne. Emperor Taizong of the Tang dynasty used this event as a pretext for invasion. Meanwhile, the kingdom of Baekje was also fighting the Silla kingdom. Baekje attacked Silla in 642 and captured 40 strongpoints. These tensions created a cycle of alliances and attacks.
The first major invasion occurred in 645. Emperor Taizong prepared a massive force for the campaign. General Li Shiji led 60,000 soldiers. The Emperor also brought 10,000 armored cavalry. A fleet of 500 ships carried 40,000 more soldiers and 3,000 military volunteers. They sailed from the Liaodong Peninsula to the Korean Peninsula. 
One of the most famous battles took place at Ansi Fortress. The Tang army arrived there in July 645. The fortress was defended by the forces of Yang Manch'un. The Tang troops attacked the walls six or seven times every day. They even tried to build a huge mound of earth to reach the city. However, the defenders captured the mound and held it. The Tang army eventually withdrew in October due to cold weather and low supplies. This failure showed that Goguryeo could defend its territory.
Later, the Tang dynasty formed a strategic alliance with the Silla kingdom. This alliance aimed to break the cooperation between Goguryeo and Baekje. In 660, the Tang and Silla armies invaded Baekje. They captured the capital city of Sabi. King Uija of Baekje surrendered at Ungjin on July 18, 660. The Tang took the king and 20,000 troops as prisoners. This victory removed one of Goguryeo's main allies.
Despite the loss of Baekje, Goguryeo continued to resist. In 661, Tang and Silla armies invaded Goguryeo from both the north and south. They were forced to withdraw in 662 after failing to take Pyongyang. However, internal problems soon weakened Goguryeo from within. In 666, the dictator Yŏn Kaesomun died. This caused a violent struggle for power among his sons. His eldest son defected to the Tang. His younger brother defected to Silla. This division caused widespread demoralization.
The final collapse of Goguryeo happened in 668. The Tang and Silla alliance launched a fresh invasion. This time, they were aided by the defector Yŏn Namsaeng. The Tang army swept away border fortifications in the spring. On October 22, 668, the allied forces captured Pyongyang. The Tang took over 200,000 prisoners and sent them to Chang'an. This victory marked the end of the Goguryeo kingdom.
The aftermath of the war changed East Asian politics again. The Tang established the Protectorate General to Pacify the East. They tried to control the former territories of Baekje and Goguryeo. This occupation lasted for about a decade. However, the Tang struggled with supply shortages. Silla had previously provided much of the logistical aid. Because of these tensions, a new conflict began. This was the Silla–Tang War, as Silla fought to resist Chinese rule over the peninsula.
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