I recently read The Imjin War: Japan’s Sixteenth-Century Invasion of Korea and Attempt to Conquer China by Samuel Hawley. The Japanese invasion of Korea and failed attempt to use Korea as a springboard for the conquest of Ming China was a text book example of how foreign wars can result from domestic politics.
At the start of the Imjin War in 1592, Japan was ruled by Toyotomi Hideyoshi, the daimyo who had succeeded in unifying the country (note that the emperor had been a figurehead for centuries). Japan had been suffering from continuous civil wars for over a century as feudal warlords, daimyo, vied for power. Hideyoshi was able to unify Japan via a combination of the carrot and stick. Some daimyo were defeated in battle. Others were persuaded to join with Hideyoshi. The rest made common cause with Hideyoshi due to a combination of threats of invasion and bribery. Hideyoshi was adept at using the confiscated lands of defeated daimyo to distribute among others to purchase their support.
Thus, Hideyoshi became the de facto ruler of Japan as a first among equals of a coalition of daimyo. At this point he was faced with two main problems. First, the ambitions of the daimyo had not been extinguished. There was always the constant threat of the creation of a faction to rebel against Hideyoshi and topple him from power. Also, Hideyoshi had not produced an heir, making the renewal of civil war almost inevitable upon his death.
Hideyoshi devised a clever scheme to consolidate his power and buy time for producing an heir. This scheme resulted in the Imjin War. Hideyoshi declared that he would conquer Korea and use it as a base for the conquest of Ming China. Then, he would conquer south east Asia and subsequently march ever westward. While this seems delusional, one has to realize that most of it was propaganda to increase the prestige of Hideyoshi. The real point of this declaration was to fire the imagination of the daimyo for acquiring riches via foreign conquest. By getting them aboard, Hideyoshi could readily accomplish his most pressing concerns: how to neutralize the armies of the daimyo and turn the acquisitive thoughts of the daimyo away from contemplations of revolts.
Additionally, an invasion of Korea and subsequently China would result in substantial casualties, thus weakening the armies of the daimyo. With the daimyo busy fighting overseas, Hideyoshi would have time to produce an heir.
Were the daimyo fools to agree to this scheme? No. While we can dismiss Hideyoshi’s dreams of conquering all of Asia, the conquest of Korea and China were certainly possible. The Japanese had a dim view of the valor and fighting skill of Koreans. With the exception of the naval squadrons led by Yi Sun-sin, one the greatest admirals in history, the Japanese were fully vindicated by the abysmal performance of Korean military forces during the war. Also, at this time, Ming China was clearly in decline. In fact, the Ming dynasty fell to the Qing dynasty a half century later. From the viewpoint of the daimyo, they were presented with a winnable war in which they would be able to immensely enrich themselves and possibly even carve out their own kingdoms.
From a military standpoint, the war was a failure for Japan. Most of their navy was destroyed, ~50,000 men died, their armies never reached China, and they failed to hold any land in Korea. For the daimyo, it was a reasonable calculated risk that happened not to come to fruition. For Hideyoshi the war was as great of a political success as he hoped. As he died a few months before the end of the war, he was able to accomplish as much of his agenda as possible. The daimyo did not revolt, he produced an heir (although there were whispers about the child’s legitimacy), and he was able to pressure the daimyo into recognizing his heir and the right to inherit his power. While it is true that within a few years of the end of the Imjin War another civil war broke out, Hideyoshi did all that he could do to prevent it, knowing that his death while his heir was a minor could lead to such an outcome.
Thus we see the Imjin War as an excellent example of how domestic political considerations can lead to foreign wars.