Yu Geun was an excellent scholar in writing as well as a diplomat dealing with international negotiation in the rapidly changing times of trouble during the reign of king Seonjo. He solved the problem of provisions for the army that was the most worrying thing during Japanese invasion of 1597 and made his best efforts in restoring Joseon’s stance between Japan and Ming dynasty.
He made active diplomatic activities before and after Japanese invasions of 1592 when the international relation took the important part. In 1580 and 1587, he worked as an officer of Seonwisa whose role was charging of Japanese envoys. At that time Hyeon So, who came as a Japanese envoy to detect Joseon, was excellent in poetry and prose. It was said that he was surprised at Yu Geun’s poetry and prose and recognized his ability. Even though there is not concrete reveal of Yu Geun’s view about Japan, it seems that he would suggest the positive preparation against Japan.
When Japanese invasion of 1592 took place, he received a Chinese envoy, who came to detect whether or not Joseon had intention of invasion, and informed him of the stance of Joseon. It was that Joseon had not intention of provoking a war by holding secret communication with Japan and did not want the army of Ming to be stationed in Joseon. After that, when the reinforcements from Ming came and Seoul was recovered, he visited China as a deputy delegate of expressing gratitude to appreciate this. Even though there was somewhat difficulty in informing China of the situation of Joseon, he did his best as an envoy. In 1601, he visited China as an envoy for the affair of reinforcing relations between Joseon and Japan and then informed it of Joseon’s will of reinforcement. During the peaceful period after Japanese mvasion of 1592, he received a Chinese envoy and let him know high level of Joseon poetry and prose by revealing his full writing ability. At that time he was known as a person versed in China.
During Japanese invasion of 1597, Yu Geun took the charge of transporting provisions for the army provided from Ming. In the early stage of that period, there was difficulty in transporting provisions from Ming because they were made via land road but he solved the problem by using marine transportation. He made ships personally for the transportation of provisions and made it easy by encouraging sailors. And also he made large ships with thick wood so that the transportation of provisions was available even in winter. Owing to his contribution to the transportation, it was possible to provide provisions for the army in the battlefield even in the difficult situation.