Literally "warring states period" (in conscious imitation of the ancient Chinese era), the Sengoku period of Japanese History (1467-1573) encompasses a total collapse of centralized authority, followed by a long period of consolidation and reunification.
Historians mark the beginning of the Sengoku period with the start of the Onin War of 1467-1477, when leagues of regional governors (known as "shugo") battled it out on the streets of Kyoto for control of infant shoguns, dramatically eliminating the last vestiges of actual power held by the once-mighty Ashikaga shogunate. The old jito and shugo hierarchy that had been initiated by Minamoto Yoritomo nearly four centuries before was now without its head, and soon disintigrated completely.
Into this power vacuum stepped a new breed of self-made man - the "daimyo" - an enterprising individual who knew or learned how to play by the new rules - all power to he who can take it and keep it, by whatever means necessary. This was something new - previously in Japanese history, all power had been granted from above as part of a trickle-down hierarchy in which all legitimacy could ultimately be traced back to the Emperor. Now all power flowed forth from the blade of a sword, could be siezed by just about anybody with the wits and the luck to do so. Some daimyo were former shugo, but many others were commoners who rose from the lowest ranks. This phenomenon of retainers rising up to the loftiest positions of power became known by the catchphrase gekokujo (下克上), literally "the low overcoming the high."
In the early years of the Sengoku, other loci of power, equally self-made, challenged the daimyo for dominance - peasant leagues (ikki), powerful shrines and temples (most notably Ise Shrine and Enryakuji on Mount Hiei), salvationist religious cults (the "True" Pure Land cult of the Ikkō Ikki) - but in the end it was individual daimyo, with their authority concentrated in the hands of single men, who subdued all comers, divided the land into personal fiefs, and battled each other for dominance.
The Sengoku period was truly an age of constant warfare, and there was almost always fighting going on somewhere in Japan at any given time. That noted, however, it is easy to overestimate the extent of the chaos. Many regions experienced years, even decades of peace at a time. Moreover, the distribution of power from a single center in Kyoto to many locales across the land, wherever a daimyo made his capital, led to a spread of courtly culture across the land, as each daimyo competed to make his castle town a "little Kyoto" in its own right. Competing in the arts as well as on the battlefield, the daimyo poured massive amounts of resources into artistic and cultural pursuits, and several art forms developed significantly in the Sengoku period and the years immediately following.
As traditional power centers fell like dominoes in the early Sengoku period, Japan devolved into a patchwork of hundreds of independent domains, and the map lines shifted back and forth for decades as the winds of fate and fortune shifted. But overall the gradual trend was increasingly toward consolidation, as the strongest, smartest, and luckiest won out and increased their holdings, until finally in the mid-16th century a mere handful of daimyo controlled nearly all of Japan. It was at this time that the first Europeans arrived, beginning with the Portuguese and Spanish Jesuit missionaries and followed by traders of all strips. The Europeans introduced a new player onto the scene, as well as a powerful new weapon - the matchlock gun.
It was this new weapon, as well as large doses of cunning and ruthlessness, that allowed one of the daimyo, Oda Nobunaga, to win a vast hegemony over central Japan. Finally, in 1573, Nobunaga abolished the long since entirely symbolic Ashikaga shogunate, signifying at long last the presence of a new central authority after more than a century of distributed power, and intitating new Azuchi-Momoyama Period - the era of the hegemons.