Encountering of Imjin War at the early time of the war, the Joseon Dynasty had its national power so vulnerable that it hardly had any strength to go through the war.
Politically, the Joseon Dynasty was on hard situation of normal operation of political affairs due to its four major disasters from King Yeonsangun to King Myeongjong, along with the continued political conflicts, in-depth struggle among the factions after the inauguration of King Seonjo and others.
Militarily, the national defense system established in early time of Joseon was collapsed, and as a way of preparing for foreign invasion, there was an agency named 'Bibyeonsa' for controlling the national affairs but it was not also able to assert its normal functions.
In addition, person like Yi Yi who advocated the principle of 'the Development of Hundred Thousand Soldier' for preparing the foreign invasion, but it was not fruitful with vulnerable national finance.
As such, the Joseon society was become vulnerable and there was no fundamental national defense scheme at all.
At such time, Japan began to encounter for new changes taking place then. In other words, Japan at the later part of the 15th century had the European merchants to develop the new commercial cities that threaten the existing feudal controlling system. Under such a situation, Toyotomi Hideyoshi took power to control the confusing time and unified the country and focused on strengthening of the feudal control. Thereafter, Toyotomi who successfully unified the country fell into the illusion to bring out the powerful force from the long struggle to seek the unification and safety in Japan while controlling the newly emerged power by invading the continent.
For a cause of Imjin War, there is an opinion that it was simple desire of Toyotomi Hideyoshi to gain his name, exaggerated illusion, desire for conquer and others while there are theories that Toyotomi succeeded the intent of Oda Nobunaga, Japan attempted to trade with Ming Dynasty but Joseon refused to cooperate or Ming Dynasty ordered Joseon not to let Japan pass through, death of his first son, Tsurumats, to break out the war, great task to have the war in the process of domestic unification, and dissolve the discontents of remaining forces and others.
Thereafter, Toyotomi Hideyoshi sent Sou of Tsushima to Joseon in March 1591 for 'Gadoipmyeong', in other word, they wish to go to Ming Dynasty and ask you to give ways. The original demand of Hideyoshi was 'Jeongmyeonghyangdo', namely, 'we will conquer the Ming Dynasty and guide us the way', but Sou changed the phrase for it would not be acceptable. Toyotomi Hideyoshi thought of conquering of Joseon as well as China, and he persisted up to the final phase of negotiation for truce was the allotment of land on southern part of Joseon. Ultimately, the biggest goal of Toyotomi Hideyoshi was in its expansion of national territory.
At the time, Senkoku Daimyo of Japan was to control the vassals by having the military mobilization system at all times for new wars. And, it did its best to share the newly acquired land from the war to the vassals and the subordinates would aim for such reward to devote their royalty to their master. Toyotomi also had the goal of national unification as Senkoku Daimyo. However, after unifying the country, this type of mechanism lost the functions, and accordingly, they attempted to revive the mechanism in new territorial wars, and it was epitomized in the Imjin War.