Using a Carp
It was in 28 when foreign aggressors invaded Koguryo (277 B.C.-A.D. 668).
The aggressors, scores of thousands strong, crossed the border and rushed to Winaam Fort, which was one of the bulwarks in defence of the capital of the country. They, however, met with Koguryo troops' stout defence. So they took the tactic of making the troops and other people in the fort surrender under stress of thirst by besieging it for a long time.
The fort stood on a rocky area, and there were few springs. The fort’s source of water supply was a pond in it. But the water began to decrease owing to a prolonged drought.
King Taemusin of Koguryo, who reigned from 18 to 44, became quite uneasy. If the situation continued the people in the fort might collapse before the enemy, the king thought.
At the time Ul Tu Ji, one of the top government officials, proposed the king a trick he had conceived. Ul said: According to our agent, the enemy is waiting for water in the fort to dry up, seeking to launch an attack when our people break down under stress of hunger and thirst. We know the enemy troops are also exhausted as they have come a long way without eating properly owing to our tactic of leaving the fields empty while defending the fort. So, if we make them "recognise" that there is plenty of water in our fort, they will withdraw realising that theirs is an absurd tactic.
Then Ul asked the king to allow him to catch a carp from the pond and take it to the enemy commander along with wine.
The king approved the proposal.
Ul caught a carp from the pond, wrapped it with a water plant and sent it to the enemy commander along with wine and a letter signed by the king.
Seeing the carp writhing in the water plant the enemy began to vacillate because they "knew" that there was plenty of water in the fort.
As long as there was plenty of water in the fort it would be unreasonable to wait for its people to collapse under stress of thirst.
The enemy commander soon decided to withdraw and wrote a letter to the king of Koguryo.
The letter read, "My king, without thinking of my stupidity, ordered me to bring troops to reprimand Your Majesty. So I had to cross the border of your country and have come here. But I've been at a loss to know what to do. Then, fortunately, I've received the food and the letter associated with your sincerity, and I'm struck with boundless joy by your quite mild and polite words.
Now that I have what I should say to my king, I'll return."
The enemy force finally began to retreat.
At the sight the king of Koguryo said, “I’m sure there is no parallel like this tactics in history. What a wonder it is to repulse such a powerful enemy by using a carp! Ul's stratagem deserves Heaven’s praise."
The Koguryo troops, in high spirits, continuously made surprise attacks on the retreating enemy at all points, inflicting wholesale deaths upon them.