After the Onin war in the mid-late 15th century, the centralized power of the Ashikaga Shogunate collapsed, leaving the field open to anyone ambitious and powerful enough to make a grab for power. During the first half of the Sengoku period (approximately 1477-1560) there was massive consolidation as daimyo across Japan solidified their power bases and battled for land and resources. The lack of central government left individual clans to fend for themselves, and in the ensuing chaos many would rise and fall in epic battles that anyone familiar with the pop-culture representations of the Samurai in Movies and Anime would recognize.
Mentioned in this podcast:
Morillo, Stephen. Guns and Government: A Comparative Study of Europe and Japan Journal of World History, Vol. 6, No. 1 (Spring, 1995), pp. 75-106 http://www.jstor.org/pss/20078620
Neilson, David Society at War: Eyewitness Accounts of Sixteenth Century Japan PhD Dissertation University of Oregon, 2007 http://gradworks.umi.com/32/85/