Why did Lu Bu sneak up on Liu Bei?

In the first year of Xingping (194), when Cao Cao attacked Tao Qian and entered Yanzhou, Lu Bu contacted Chen Gong, and when Liu Bei fought with Yuan Shu, he attacked Xuzhou.

In the second year of Xingping (195), Yuan Shu led an army to attack Xuzhou and fought Liu Bei in Xuyi and Huaiyin. After a month of stalemate, both sides won and lost. ? Yuan Shu wrote to Lu Bu, promising to send 200,000 meters of welcome guests to induce him to attack Xiapi. So Lu Bu landed in the east. When the army reached forty miles west of Xiapi, Liu Bei's corps commander Danyang sent Sima Zhang to meet Lu Bu, and revealed to Lu Bu that Zhang Fei and Cao Bao were fighting in secret. There was chaos in Xiapi City, and Danyang soldiers waited for the arrival of Lu Bu in Xibaimen City, so Lu Bu made great strides and arrived at the gate in the morning.

After dawn, Danyang soldiers opened the gate, and Lu Bu sat on the gate, commanding the army to crush Zhang Fei and capture Liu Bei's wives and children and their families. At this time, Liu Bei was defeated by Yuan Shu and fled to Haixi. He was hungry and tired, and asked Lu Bu to surrender. Lu Bu was very angry that Yuan Shu stopped transporting grain, so he prepared chariots and horses to meet Liu Bei, made Liu Bei the secretariat of Yuzhou, and sent him to Xiaopei. Lu Bu claimed to be a herdsman in Xuzhou. At this point, Xuzhou has been captured by Lu Bu.

Extended data:

In the third year of Jian 'an (198), Cao Cao personally led an army to attack Lu Bu, and the army arrived at the gates of Xiapi. Cao Cao besieged the city for three months, and his subordinates Hou Cheng, Song Xian and Xu Wei rebelled, which bound Chen Gong to surrender. Lu Bu saw Cao Jun in a hurry on the White Gate Tower, and the tide had gone out. So he ordered the head to be handed over to Cao Cao, and December was unbearable.

References:

Baidu Encyclopedia-Lu Bu