The Iron Kingdom Goguryeo: How It Defeated the 'Greatest Emperor' Tang Taizong

History has figures worthy of being called the ‘GOAT (Greatest Of All Time)’. If you had to pick one across all of Chinese history, many would name Tang Taizong Li Shimin. He was an emperor who seemed maxed out in military, political, and diplomatic skills. 🎮

He embarked on the greatest “raid” of his life. The target was the tenacious northern kingdom, Goguryeo. But what a surprise! Instead of facing a final or even mid-level boss, he was stopped and forced to retreat by a single small fortress. Today, we delve into the hidden details in historical records to uncover why the mighty Tang Taizong had no choice but to fail in the Goguryeo campaign, revealing this fascinating story.

1. The Most Glorious Justification, the True Desire for Steel

In 645, Tang Taizong declared at the departure ceremony, “I lead this army not for Goguryeo’s land or wealth, but to save the people and punish the traitor Yeon Gaesomun, who assassinated the king!” 📜 What a powerful justification!

When asked why he killed the king after seizing power in a coup, Yeon Gaesomun replied boldly, “The old king was unvirtuous, so the ministers deposed him. What does this have to do with Your Majesty?” Essentially saying, ‘Hands off our internal affairs.’

Enraged, Tang Taizong mobilized hundreds of thousands of troops, but his real intention was different. It was the source of Goguryeo’s strength—the shining ‘steel (鐵)’.

2. The Northern Forge, a Kingdom with a Steel Heart

The Goguryeo army Tang faced was on another level. The Gaemamusa (Armored Horse Warriors), clad head to toe in iron armor, were a shock. Arrows from Tang soldiers simply pinged off their armor helplessly.

This was not just bravery. Goguryeo was a massive state-run ‘military factory.’

The Secret of Goguryeo Steel

Goguryeo’s metallurgists produced large quantities of steel by mixing carbon with iron, creating weapons and armor that were both hard and flexible. This meant every soldier was equipped with top-tier arms and armor for the time.

The Moving Iron Fortress Army

Equipping 5,000 Gaemamusa required over 300 tons of iron. The entire industrial system of mining, smelting, and crafting this iron into armor was Goguryeo’s true weapon. Tang Taizong’s target was this ‘steel heart.’

3. The Most Dramatic Siege in History: Ansi Fortress

At the start of the war, Tang’s forces were unstoppable. The only obstacle to the emperor’s path was the small, seemingly outdated Ansi Fortress. But no one expected this small fortress would become the massive trap that would hold back a great emperor.

① Beating the Drum at the Emperor’s Banner

Tang Taizong planted his yellow banner symbolizing his authority in front of the fortress to intimidate. Normally, this would cause surrender, but the commander of Ansi led troops out onto the walls each time the banner was visible, beating drums and shouting war cries. 🥁 It was a psychological warfare and provocation that said, “Emperor? Not scared at all.”

② The 60-Day Miracle: The Earthen Mound Siege

When all attacks failed, Tang Taizong mobilized 500,000 troops over 60 days to build a massive earthen mound higher than the fortress walls, thinking the siege would end from this vantage point.

But then, as if by a miracle, heavy rain caused one side of the mound to collapse against the fortress wall. Seizing the opportunity, hundreds of Goguryeo warriors rushed out and quickly captured the mound. The very stronghold Tang had painstakingly built became Goguryeo’s frontline fortress.

③ The Emperor’s Silk Gift to the Defeated

After more than two months of siege, Tang Taizong decided to retreat. But instead of leaving empty-handed, he sent 100 bolts of silk to the Ansi commander with the message, “You have loyally defended the fortress and served me well; I am pleased to present you this gift.” This was one of the most unusual and impressive gestures of respect in history, sent to the general who had soundly defeated him.

The Emperor’s Tears and Final Testament

Worse than the defeat at Ansi was the retreat. It was winter, and the army froze and starved in the marshes of Liaodong. Emperor Li Shimin himself carried earth to build roads in a desperate situation.

This humiliating failure became a lifelong trauma. Years later, on his deathbed, he left this final instruction to his crown prince:

“Never invade Goguryeo again.”

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This last confession from a conqueror who ruled the world was not mere regret. It was a sober recognition that Goguryeo’s steel armor, strong fortress, and indomitable spirit could not be broken even by imperial might. The cries from Ansi Fortress echo through a thousand years, showing us that with thorough preparation and unyielding will, miracles can happen even against overwhelming force.