In 1592, when the Imjin War occurred, Yi was Mobile Border Commander. On 24 April 1592, Yi fought against Japan at the Battle of Sangju. Yi organized a unit about 800 soldiers but they were unstructured. Yi started to train with the rest of them on 25 April. They were defeated by Japan being ambushed. Yi had run off to Chungju and met Shin Rip. Shin Rip tried to kill Yi taking responsibility for the defeat, but Yi wasn't killed because Kim Yeo-mul stopped Shin Rip. Yi participated in Battle of Chungju with Shin Rip and he was defeated and pulled back to Pyongan Province once again. Yi contributed to recapture Pyongyang with forces of Ming by winning Siege of Pyongyang on 8 January 1593.