Oda Nobunaga (1534-1582) was the first individual to attempt to unify Japan at the end of the Warring States period; his ultimate goal, though he never realized it, was to bring all of Japan "under a single sword" (tenka-fubu ). Like so many others in the history of Japan, he rose from an obscure family through ruthless ambition to become one of the most powerful men in Japan. His rise to power was slow and deliberate and his use of power unforgiving. The most significant step he took in unifying the country was the destruction of the Buddhist monastery of Mt. Hiei. All throughout the medieval period in Japan, from the Heike war onwards, the monks of Mt. Hiei had played a significant role in both the political and military course of Japan. Seeing Mt. Hiei as a threat to future stability, he destroyed the monastery and hunted down every single Hiei monk and slaughtered them, regardless of their age or innocence.
Perhaps one of Oda's most significant contributions to Japanese history, outside of laying the groundwork for the future unification of the country, was his eager embrace of Westerners. Perhaps out of his dislike of esoteric Buddhism, he was fascinated by Christianity and welcomed Jesuit missionaries. As a result, he's the first Japanese leader to appear in Western histories. He also, very shrewdly, embraced Western technology firearms, in particular. Firearms had been imported into Japan since the late fifteenth century; although these weapons were primarily firelocks and inherently unstable (you can't use a firelock in the rain or snow, for instance, and they have a disturbing tendency to blow up in your face), Oda Nobunaga was the first Japanese to figure out both offensive and defensive tactics with the new weapons. Besides retraining his armies for new tactics, he also built massive stone forts that would resist the new firearms. Finally, he was the first Japanese leader to employ iron-cladding on his warships, which made them virtually unbeatable.
Oda never succeeded in unifying the country; just as he was on the verge of success, he was assassinated by two of his generals at the age of forty-eight. Although he had eliminated the wild card of the Mt. Hiei monks, there remained much to do. The hardest task would be to restructure the country to guarantee a lasting peace among the warring feudatories. That task largely fell to the hands of Toyotomi Hideyoshi, the "Wealth of the Country."