The silent, black-clad ninja who spies, sabotages and assassinates—without leaving a trace—remains a popular Japanese character in modern books and films. It has loosely inspired pop-culture phenomena ranging from Teenage Mutant Ninja Turtles to the American Ninja Warrior. But facts about ninja history can be just as elusive as the iconic fighters themselves.

Some modern scholars question whether ninjas actually existed—or were merely a mythic invention. That skepticism stems, in part, from ninjas often being described as martial arts experts with supernatural abilities, or as sorcerers who can conjure fire at their fingertips and move wind and objects with hand signals. In many stories, they fly and even split themselves into multiple bodies to foil those in hot pursuit.

Most scholars believe that historical accounts of ninjas, like those of many underworld characters, were wildly embellished, while retaining a grain of truth. “The usual approach, even among scholars, is simply to accept the original ninja myth as a genuine historical phenomenon that has for centuries been greatly romanticized and, more recently, highly commercialized,” writes Stephen Turnbull, a Japanese history expert and author of Ninja: Unmasking the Myth.

Ninjas were active from the 14th century, when they were hired by daimyo, or feudal Japanese warlords, chiefly for intelligence and counterintelligence. But their intrinsically secretive nature left few mentions of them in the historical record. Much of what’s known comes from texts written in the 1600s and later, well after the shogun wars, when ninjas flourished.

What distinguished a ninja? Unlike Japan’s other famous warriors, the samurai, who were highly trained fighters from elite families, ninjas came from all levels of society. And unlike samurai, ninjas weren’t bound by a strict code of honor (bushido) that required face-to-face fighting. Warlords could employ ninjas to engage in the kind of guerrilla warfare that would dishonor a samurai.

Since they served as mercenaries and spies, ninjas needed to be especially adept at disguise and subterfuge. And while popularly depicted as trained assassins, they were more likely to marshal skills of stealth, distraction and counterintelligence than to kill. Their ultimate responsibility was to covertly gather useful intelligence for their lord.

The word “ninja” doesn’t appear in historical texts and testimonials before the 19th century. Rather, early texts referred to these fighters most commonly as “shinobi,” which shares a common character with ninja in Japanese kanji (writing). The Vocabulario da Lingoa de Iapam, a Japanese-Portuguese dictionary published by the Jesuit mission in Nagasaki in 1603, defines shinobi as “a spy who in times of war enters a castle by night or clandestinely, or infiltrates the enemy ranks to obtain intelligence.”

That intelligence made shinobi exceedingly valuable to their patrons, says historian Yamada Yüji, vice president of the International Ninja Research Center at Japan’s Mie University and editor of a multidisciplinary anthology of ninja studies: “You need to know the topography of the enemy’s position, the condition of his food supplies, the structure of his castle. It was the job of the shinobi to obtain this kind of crucial information. They would infiltrate the enemy domain and ascertain the lay of the land…and create chaos through acts of sabotage and arson.”

As mercenaries, ninja fought for warlords all over Japan. But according to the Gunpo Samurai Youshuu, a dictionary of samurai martial law, the best feudal-era shinobi came from the neighboring provinces of Iga and Koka, situated in the mountain region southeast of Japan’s then-capital, Kyoto. By the 14th century, about two dozen ninja schools had popped up throughout Japan. According to the 17th-century Bansenshukai, a 22-volume encyclopedia on the art of the ninja, the ninjutsu discipline found inspiration in the guerrilla tactics of the brilliant Chinese war strategist, Sun Tzu.

The Bansenshukai describes the ninja arts as social skills, conversation techniques, mnemonics (memory aids), transmission techniques, medicine, astronomy and even sorcery. Ninjas were trained to use their intellect and vast knowledge to infiltrate any social setting, gain knowledge and escape safely to report their findings to their patron. Psychologically, a good ninja required intense self-discipline and purity of mind, says Yüji: “a mental state of absolute tranquility, in which a person will not flinch even if a bare blade is pressed against his chest.”

As masters of disguise, ninjas would often infiltrate their target not under cover of night, but in broad daylight, disguised as a merchant or a Buddhist priest. They used many common tools, such as the sickle and sword, as weapons so they could blend in with peasants and farmers. But they also famously carried shuriken, the ninja star, because these pocket-sized, hand-held throwing blades could be easily hidden and used to disarm an opponent.

While their origins may reach back as far as the 12th century or before, shinobi were active when Japan was rife with territorial skirmishes between warlords. Shinobi played important roles in the Nanbokucho Wars (1336–1392) and the Warring States Period (1467-1568), among others.

The Honno-ji Incident of 1582 shows how ninjas could even influence the course of Japanese history. After a samurai general assassinated Oda Nobunaga, one of three powerful shoguns who sought to unify Japan, at the Honno-ji Temple in Kyoto, he then set about to kill Nobunaga’s loyalists and allies. But his target, Tokugawa Ieyasu, another of Japan’s “great unifiers, ”was lucky to have as a friend and general the ninja Hattori Hanzo of Iga. A master swordsman, Hanzo (or possibly another, anonymous ninja) is believed to have smuggled Ieyasu out of enemy territory and brought him safely home.

Had Ieyasu been killed, Japanese history might have veered in a substantially different direction. In 1603, the emperor Gō-Yozei elevated him to be a shogun. Ieyasu’s Tokugawa shogunate, the last in the shogun era, is credited with ushering in two centuries of peace and prosperity known as the Edo period.

The transition to peace was messy, however, and marred by enormous wealth inequality. One ninja, Iga-born Ishikawa Goemon, is said to have attempted to make life more bearable for the peasants by using ninjutsu to steal gold from the rich and give it away to the needy. For some, Goemon represents a ninja gone rogue. Authorities such as the Japan Ninja Council, however, consider him a legendary outlaw hero likely to have sprung from someone’s imagination, and not a real historical figure.

Whether real or fictional, Goemon’s story ends tragically. After he failed in his attempt to assassinate Toyotomi Hideyoshi, a powerful warlord, Hideyoshi’s men executed Goemon by boiling him alive. In depictions on many silkscreens and woodcuts, Goemon’s young son was also thrown into the tub, and the father heroically holds him above the boiling oil, saving his son’s life as he himself dies.

With such colorful stories, it’s not surprising that ninjas became such pop culture icons for Japan. A mayor of Iga, Okuse Heishichirō, played a large role in bringing ninjas to broader public consciousness in the 1950s by building the Ninjutsu Wonder Museum. A flood of books, articles and pulp fiction followed.

A key popularizing moment came with the ninja scene in the 1967 James Bond film You Only Live Twice. Just as hundreds of ninja assassins abseiled, commando-style, from a shattered skylight into SPECTRE’s lair, throwing shuriken, the myth of the ninja had broken through and infiltrated mainstream pop culture. By the 1980s, ninjas had exploded into a huge business, appearing widely in movies, TV, video games, toys, comics and even product naming—from blenders to motorbikes to perfume. The Teenage Mutant Ninja Turtles became a hit animated series, and its related toys became some of the best-selling action figures of all time. It’s the kind of widespread fame and attention that, ironically, the shinobi actively shunned.