Stephen Turnbull 
Samurai Armies 1550 1615 [PDF ebook] 

In 1543 three Portuguese merchants entered a turbulent Japan, bringing with them the first firearms the Japanese had ever seen: simple matchlock muskets called arquebuses. They proved a decisive addition to the Japanese armoury, as for centuries the samurai had fought only with bow, sword and spear. In 1575, one of the greatest original thinkers in the history of samurai, Oda Nobunaga, arranged his arquebusiers in ranks three deep behind a palisade and proceeded, quite literally, to blow his opponent’s cavalry to pieces, marking the beginning of a new era in Japanese military history.