The sengoku jidai was a period of great unrest and confusion. The central government of the country was weak, and breaking down. Since they were not held in control by any central power, each of the regional daimyo tried to extend their own territory and power by going to war with their neighbors
This confusion was brought to an end by three remarkable men -- Oda Nobunaga, Toyotomi Hideyoshi and Tokugawa Ieyasu -- who, together, managed to reunite the country and break the power of the individual daimyo.
Nobunaga was a very determined, strict and merciless man who had both a talent for leadership and a good military mind. He defeated his local rivals one by one, steadily expanding his control until he dominated almost half of Japan, and had defeated all of the most serious threats to his power. Unfortunately, he died before he could unite all of Japan. One of his generals, Akechi Mitsuhide, betrayed and killed him when he was just 49.
Another one of Nobunaga's generals, Hideyoshi, revenged his former master by killing Mitsuhide, and then went on to defeat all of the remaining independent daimyo. Hideyoshi also was a brilliant general, but his character was much different than that of Nobunaga. He was less stern and strict. He also was a good administrator, and he laid the foundation for a new government of Japan.
When Hideyoshi died, two groups of opposing daimyo fought to see who would inherit control of the country. Tokugawa Ieyasu was the victor. Although Ieyasu was not as great a general as either Nobunaga or Hideyoshi, he was brilliant at administration and good at motivating his subordinates. By the time he finally defeated the other daimyo at the great battle of Sekigahara, he had already established a powerful government in Edo. With all opponents defeated, Tokugawa Ieyasu became Shogun and he established Edo as the center of Japan's government, society and culture for the next 250 years.