The basic line of foreign policy that the Joseon Dynasty adhered to was to maintain relations with the Ming Dynasty as a tributary state and maintain friendly relationships with Japan and Yeojin. During the King Taejong’s reign, the 3rd King from 1397 to 1450, the Joseon Dynasty opened the Ungcheon port. During the King Sejong’s reign, three ports were opened. They were Busanpo, Naeipo and Yeompo. In 1510, there was commotion in these three ports caused by the Japanese, who resided there, rioting. The government justly cracked down the commotion. The Japanese were exiled from three ports and the relations between Korea and Japan were terminated. .

Japan continued to request amity. The Joseon Dynasty opened only the Naeipo port to the Japanese resulting in maintaining friendly relations.
According to the political changes in Korea and abroad, the Japanese invasion that had run rampant, since the end of the Goryeo Dynasty, seemed to be slightly relieved during the early period of the Joseon Dynasty. The Japanese invasion was continued intermittently in roughly ten-year cycles. These cycles includes: the invasions of 1523, the 17th and 18th year of King Jungjong’s reign, the Japanese invasion at Saryang in 1544, the 39th year of King Jungjong’s reign and the Japanese invasion of 1555, the 10th year of King Myeongjong’s reign. Since the Japanese invasion of 1555, the diplomatic relations between the Joseon Dynasty and Japan were basically non-existent up until the Japanese invasion of 159.2 The Joseon Dynasty put never any attention into the domestic situations of Japan.
The royal court of the Joseon Dynasty made a new battleship called The Panokseon. It was made in order to handle the Japanese invaders. It played a significant role during the Japanese invasion in 1592. The government accelerated the development of firearms, including large guns in addition to The Panokseon, during that period.
Oda Nobunaga set the foundation for unification through the war amongst rival warlords during the age of civil war, in Japan. In 1590, his follower Akechi Mitsuhide assassinated Nobunaga. right after Toyotomi Hideyoshi unified Japan.
Hideyoshi became Kanpagu in 1585. He pursued the unification project of Japan and exerted power that was equivalent to the emperor, taking the seat of the grand minister in 1586. While he held the power in the central government, he considered the independent powers in local areas to be a serious barrier. He strived to devise a way to diminish the powers of The Daimyo, which were the feudal lords.
He needed a good cause to avoid an exhaustive armed conflict amongst the domestic powers. He planned an expedition army to attach the Ming Dynasty and tried to mobilize and concentrate the military forces of the Daimyo.
Japan exchanged western cultures by opening the Dongjin Port to western forces such as, Portugal and Spain. The Portuguese delivered the iron firelock gun and the infantry, armed with firelocks, were recognized as more important elements than the previous army, which was comprised of Samurai, who were clever in battle using short arms. The Society of Jesus also incurred substantial changes in Japan’s society. Devoted believers, such as Konishi Yukinaga, were brought up. About 2,000 soldiers that invaded the Joseon Dynasty were baptized.
Hideyoshi continued his plan to attack other countries. In Jan. 1591, he assigned a number of provisions, battleships and soldiers all over the country. He installed the headquarters at Nagoya, a fishing village in Kyushu, and built a castle as the cornerstone for the invasion of the Joseon Dynasty.
The following January, he completed the compilation of an amphibious attack unit and in March he reorganized it. The land forces totalled 158,300 from the first attack squadron to the 9th attach squadron. The remaining squadrons in Japan included: the Nagoya, which was the first squadron led by Konishi Yukinaga, the second squadron led by Kato Kiyomasa and the third attach squadron led by Kuroda Nagamasa. Guki Yositaka, Wakizaka Yasuharu, Kato Yoshiaki and Todo Takadora were organized as the naval forces. The attack unit, of the Joseon Dynasty, arrived in Tsushima in a quick succession under a careful attack plan. Hideyoshi gave strict orders that “there must not be any damages, even one horse, while crossing the sea”. The Japanese attack unit waited for the day of the invasion in Tsushima.
Hideyoshi requested several times prior to the invasion that the Joseon Dynasty send envoys through the governor of Tsushima. In September 1589, the Joseon Dynasty then sent envoy that included: Hwang Yun-Gil, Kim Seong-Il and Heo Seong , which was the 22nd year of King Seonjo’s reign. The envoys met Hideyoshi and returned in Jan. 1591, which was the 24th year of King Seonjo’s reign.
The reports of the envoys also illustrated the conflict between the westerners and the easterners. Hwang Yun-Gil, the westerner and the senior envoy, reported this. It is described below.
“Japan has been preparing the battleships. There must be a war.”
Kim Seong-Il, an easterner and a vice envoy, reported this in a totally different manner. He reported that there was no indication that Japan intended to invade the Joseon Dynasty and Hideyoshi was not the type of person who we worried about. Heo Seong, Seojanggwan, agreed with Hwang Yun-Gil, eventhough he was the easterner, at that time. Hwang Jin, who accompanied Kim Seong-Il, also reported the same opinions.
The reports from the envoys accelerated the disputes in the Joseon Dynasty. Countermeasures were planned far after that. The Joseon Dynasty built citadels in each province and maintained the armories. They singled out those who had distinguished abilities amongst the military officers, irrespective of governmental ranks.
The royal court ordered to concentrate on the construction of the citadel at Gyeongsang-Do and Jeolla-Do, under the assumption that the Japanese forces were better in naval battles than they were in land battles Some high officials stubbornly opposed the plan insisting that the Japanese forces would never invade. Ordinary people mobilized for the maintenance of citadels, or military drills. They had a grudge against the local governors or soldiers who commanded them. The citadel maintenance was carried out in a perfunctory manner. People who hadn’t had any invasions from the outside for around 200 years made the war preparation. Some of the preparations were very loosely made. Nevertheless, according to the selection of prominent personnel irrespective of official ranks, the greatest benefits were the selection of Yi Sun-Shin, Gwon Ryul and Won Gyun.
Since its foundation, The Joseon Dynasty lived in a peaceful ear for about 200 years. It never thought that Japan would invade by crossing the sea with all its forces. As a result the Joseon Dynasty neglected the Japanese invasion.