Narrow Road To The Deep North 6

‘When I tell you, that war is the foundation of all the arts, I mean 
  also that it is the foundation of all the high virtues and faculties of 
  men. It is very strange to me to discover this, and very dreadful, 
  but I saw it to be quite an undeniable fact.’ 
                                        John Ruskin, The Crown of Wild Olive

Like most bloodbaths, it was caused by money. The hundred years of continuous social upheaval, political intrigue and military conflict began with the failure of the Ashikaga Shogunate in Kyoto, successor to the Kamakura Hōjō, to win the loyalty of more remote daimyô local warlords. Improvements in agriculture had brought the promise of prosperity, trade with China had brought cash, and earthquakes and famine had brought the armed insurrection of farmers, weary of debt and taxes.
A dispute over shogunal succession brought the final straw, the Ōnin War, and the beginning of the Sengoku jidai ‘Warring States’ century. Fighting in and around Kyoto lasted for eleven years, leaving the city almost completely destroyed. The conflict spread to outlying provinces, culminating with a survival sequence of the three greatest warlords, Oda Nobunaga, Toyotomi Hideyoshi, and Tokugawa Ieyasu, the Wagon Days historical heroes of the country, the grand unifiers of Japan, writ large. Nobunaga made it, Hideyoshi baked it, Tokugawa ate it.
In any era of constant war, established aristocracy and bureaucracy lose out to new meritocracy. During the Sengoku, overlords were overthrown by more capable subordinates, known as gekokujō, ‘low conquers high.’ Samurai culture was infiltrated by gifted mercenaries from lower social strata, making names. Warfare tactics and technologies in the fifteenth and sixteenth centuries underwent radical improvement, sometimes to the detriment of the lone wolf swordsman. Large numbers of lightly armored ‘light-foot’ ashigaru infantry, thousands of humble ordinary people with nagayari long lances or naginata, were combined with cavalry in new maneuvers. The arquebus, a matchlock brought by the Portuguese on a Chinese pirate ship in 1543, and mass-produced within a decade, began playing a critical role. By the end of the Sengoku, hundreds of thousands of firearms existed in Japan and massive armies of those same numbers clashed in battles.