The Japanese invasions of Korea (1592–98) was largely caused by too many unemployed samurai warriors, who were largely out of jobs as peace spread in Japan. Toyotomi Hideyoshi, de facto ruler of Japan, needed to prevent disturbances by unemployed samurais and result was invasion of Korea. It's not the only reason, but a major one.
The scale of the war was massive. Japan alone brought a force of 158,800 in 1st invasion of 1592. In the 2nd invasion of 1597–98, Japan again brought similar number of troops.
For comparison in the Long Turkish War or Thirteen Years' War which was between 1593 to 1606, Ottoman Empire had 160,000–180,000 men.
Ming Dynasty sent 50,000 men to aid Korea, and some say this started the decline of Ming Dynasty.
One really great book I recommend for anyone wanting to learn about this war is
"The Imjin War: Japan's Sixteenth-Century Invasion of Korea and Attempt to Conquer China
August 12, 2014" by Samuel Hawley.
Some interesting tidbits from the book:
1. This war was the first time a large group of Christians (not Western but Japanese Christians) set foot in Korea. Of the 12 (?) forces commanded by different warlords, 2 or 3 had large number of Christians. And these were first Christians to set foot in Korea.
2. Chosun Dynasty court maintained the Annals of the Joseon Dynasty (also known as The True Record of the Joseon Dynasty), annual records of the Joseon Dynasty of Korea. They were kept from 1413 to 1865. The annals, or sillok, comprise 1,893 volumes and are thought to cover the longest continual period of a single dynasty in the world.
https://en.wikipedia.org/wiki/Veritable_Records_of_the_Joseo...
Four copies of the records were kept in geographically separate area within Chosun. However during the Imjin War, 3 of the 4 were burned down by Japanese forces. The Imjin War book talks about how the last set of copy was being transported in numerous, lumbering carts pulled by ox to keep it out of marauding Japanese forces. The convoy was in a village when Japanese forces caught up. The convoy had not enough troops to resist. They then faked it to look like the village held more Korean troops than actual, by using extra flags and other camouflage. Japanese forces stayed away and this allowed the last backup of 4 backups to survive.
This tidgit was extra interesting to me for following:
1) Faking it till making it.
2) how 4 sets of backup was just barely enough.