This is a story from the novel Three Kingdoms.
Zhuge Liang led 300,000 Shu State troops in a march towards Chencang, the vital communication stronghold（要塞，据点） of the Wei State. It was his second expedition against the Wei State. Hao Zhao, the general guarding the city dug the trenches deep and built the fortress（堡垒） strong. He waited for the attack. Everyday, Zhuge Liang sent soldiers to challenge the Wei army in battle, but the Wei troops held fast to their position and refused to come out.
One day, a scout（侦察兵） came back to report that Hao Zhao was seriously ill. Zhuge Liang thought that the chance had come at last. He ordered Wei Yan and Jiang Wei to take 5,000 troops and surround Chencang in three days. They were to attack the city when they saw a fire inside the city. After that, he gave Guan Xing and Zhang Bao secret instructions. As the health of the Wei State general Hao Zhao was deteriorating（退化，恶化） , suddenly it was reported to Hao Zhao that the troops from Shu State had arrived outside the city. Hao Zhao hurriedly ordered officers and soldiers to defend the city in their position. Unexpectedly, a fire broke out on each of the city's towers. There was chaos inside the city. Hao Zhao was frightened to death. The troops of the Shu State smashed all enemy resistance and advanced into the city.
Three days later when Wei Yan and Jiang Wei led their troops to the gate of the city, they heard somebody shouting from the top of the city wall, "You have come too late!" The two generals fixed their eyes upon the person. It was the Prime Minister of the Shu State, Zhuge Liang. The two generals got down from their horses hastily and saluted. Zhuge Liang said, "The reason that I asked you to attack the city in three days is to keep the doldiers calm. In fact, Ihad already sent Guan Xing and Zhang Bao to secretly leave the center area of Han the same night as I ordered you. By disguising myself, I hid among the soldiers and set out for Chencang by starlight. We gave the enemy a surprise attack. The fire was set on at my instruction and it threw the city into confusion. This is called 'taking the enemy by surprise and striking when it is unprepared'."