Impis are Zulu regiments, formed up by conscription of men in a certain age group. Each impi was commanded by an induna.
The men of an impi could not marry until given permission by the King. Veteran impis were granted the right to wear the isiCoco, or headring: a ring of polished wood that was woven or matted into the hair.
The Zulu encircling strategy was called "horns of the bull" or izimpondo zenkomo. It was invented, or at least refined, by King Shaka. In this strategy, the army is separated into four parts: the chest, which faces the enemy directly, the horns, which lead out on either side, trying to encircle, and the loins, which stay back and wait in reserve, with their backs to the battle.
Each impi had a characteristic uniform and shield color.
For example, younger impis might be assigned cowhide shields that were mostly black. Red shields meant an impi of medium experience, usually married veterans. Older, more experienced regiments had more white in the shield. White cattle were considered more valuable, and could only be owned by the king.
The fighting uniform was a string around the waist with strips of fur or cowhide dangling down. Specific kinds of fur, hide, or feathers could identify specific regiments. Each impi had a ceremonial dress, too elaborate and expensive to wear into battle.
The 6 foot high shield, with a tough cowhide cover over a light frame, was carried on the left arm. In the right hand the Zulu warrior carried a short stabbing assegai. Young boys would travel with the army, carrying sleeping mats and food, while the warriors, barefoot to make them tough, carried only shield and spear. Unencumbered, an impi could cover 50 miles a day, on foot.
Rivalry was intense between the impis. Before departing from their kraals for battle, the warriors would dance a leaping dance as they boasted of the enemies they would kill. Some impis had to be kept apart because of jealousy and rivalry, or the dances would turn violent.
In battle, the impis would shout the names of their regiments, or the general war cry of uSuthu!.
In 1879, there were 33 Zulu impis, seven of older veterans, eighteen of married men, and the rest fairly young. The Zulu population was estimated at 3 million, with 100,000 in the army.