中日韓三國在1592至1598年之間發生重大戰爭，明朝稱之為萬曆朝鮮之役，日本稱之為文祿．慶長之役，朝鮮稱之為壬辰倭亂，在這場戰爭中，三國各自敗傷慘重，但是三國各自稱勝。明朝為之國庫空虛，埋下敗亡的導火線，於日本則以豊臣秀吉政權結束作收，朝鮮也因此役元氣大傷，政局變動不安。 在這場戰爭中，姜沆在1597年被俘至日本，在1600年被送回朝鮮，以在這三年中的經歷撰述成《看羊錄》一書，對於當時日本的風俗、政治及學術狀態皆有所描述。在日本期間與藤原惺窩結識為友。 姜沆在日本期間與藤原惺窩結識為友，姜沆告訴藤原惺窩關於朱子學在朝鮮的發展狀況，朝鮮李退溪的朱子學逐漸傳入日本，朱子學成為日本官方學術的代表。最後，韓日發展自己的學術體系，中韓日三者在學術上各自走上自己的路線。

China, Japan and South Korea experienced a significant war in between 1592 and 1598 and it was called respectively by Chinese Ming Dynasty as the battle of WanLi Chaoxian (Former name of North Korea Chosun dynasty 1392-1910); the battle of Bunroku keicho by Japan; and Renchen Woluan (Japanese Pirate Invasion) by North Korea. During this war time, the above mentioned three countries all suffered horrendous loss despite of the fact that all claimed to win the battle: the Chinese Ming dynasty experienced international financial crisis as a result, which eventually cost its throne; the political power of the Shuijing Xiuji regime (Toyotomi Hideyoshi 1536-1598) also came to an end soon after; similar catastrophe of political upheaval also took over Chaoxian (North Korea). During this war time, Jiang Hang was captured and delivered to Japan in 1597 but transferred to North Korea in 1600. During these three years, not only did he complete a book entitled Kan Yang Lu（看羊錄）, which detailed Japan's customs, politics, and academics, but also made friends with Fujiwara. Jiang Hang informed Fujiwara Seika about Zhu Zixue's (Zhu Xi 1130-1200, also known as Master Zhu 朱子, Song dynasty Confucian writer and propagandist, founder of neo-Confucianism) development in North Korea. As a result, Zhu Zixue was proclaimed via Li Tiexi and became popular in Japan. In no time, Zhu Zixue then became the official representative doctrine in Japan in a short time. It was not until later, Korea and Japan started to develop the doctrine of their own.