It's a slightly AUish retelling of the Age of Warring States of Japan. The variations come with each person's story mode. You can play the game as one of the greatest historical figures of the time, like Takeda Shingen, Uesugi Kenshin or Oda Nobunaga. (though Toyotomi Hideyoshi doesn't become playable until you pick up Samurai Warriors XTreme Legends or Samurai Warriors 2, and Tokugawa Ieyasu isn't playable in the first and Xtreme Legends, but he is a playable character in SW2)