1. They weren't particularly effective as assassins.
Thanks to the precautions warlords took to protect against evil ninja assassins (building homes with counterbalanced floors that squeaked, hiring bodyguards to watch them sleep, and requiring every member of their households to wear pants that dragged along the floor so walking quietly became impossible), there aren't many documented cases of ninjas pulling off successful assassinations. Not to say they didn't try. In the 17th century, for instance, General Oda Nobunaga was surprised by three cannon-wielding ninjas who tried to blow him away, but missed.
2. The first one was a cross-dresser.
3. They rely on a Chinese guidebook.
While ninja assassins were almost all Japanese, ninja philosophy is a Chinese invention. It comes from The Art of War, the battle guide written by Chinese general Sun Tzu in the 4th or 5th century BCE.
4. The Japanese didn't like them half as much as your little brother does.
In the eternal debate over which is cooler—pirates or ninjas—most ancient Japanese would vote squarely with the swashbucklers. The idea of devious, back-stabbing guerrillas who attacked in secrecy conflicted with the Japanese ideal of the brave, loyal Samurai. Over the centuries, warlords came to accept the necessity of covert operations, but only grudgingly.
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