Charm and ingratiate yourself to your enemy. When you have gained his trust, you move against him in secret.

It's used to describe insidious people.

Related atory:

During the Warring States, in the year 341 BC, General Gongsun Yang led 50,000 soldiers to invade the state of Wei.

Wei's king Hui was extremely worried. Gongzi Ying of Wei said, "Gongsun Yang was originally a native of Wei and we were good friends for some years. If I could talk to Gongsun Yang, I may be able to persuade him into ordering a retreat. If we find that he cannot be trusted, we can retreat into Wu city which has impregnable fortifications."

Gongzi Ying then led 50,000 troops to guard Wu city.

Gongsun Yang arrived at Wu city and hearing that Gongzi Ying was in charge of its defence, hatched an idea.

Gongsun Yang sent a letter to Gongzi Ying which wrote, "Formerly, you and I enjoyed a good relationship. I greatly appreciate your trustworthiness and your valuing of our friendship. Why not order a mutual retreat? If you agree, let's meet outside the city in three days' time to discuss the matter."

Three days later, Gongzi Ying brought 300 unarmed troops showing his country's goodwill and sincerity. Gongsun Yang himself also brought no weapons. They started a discussion and the atmosphere was very cordial. Gongsun Yang then invited Gongzi Ying to his tent for a banquet.

Upon reaching the Qin's camps, Gongzi Ying and his 300 unarmed troops were siezed and arrested. The Qin soldiers then disguised themselves as Wei's troops and went to the city gates. They claimed that Gongzi Ying is back and asked for the gates to be opened. The Qin troops charged through the gates and conquered Wu city. Gongzi Ying was taken captive to Qin.