The Zulu Impi is perhaps the first "mechanized infantry"... they are fast defenders that will withdraw from combat if they are losing (unless fighting another fast unit).
Upon coming to power, one of Shaka's first acts was to reorganize the Zulu army. Shaka first rearmed his men with long-bladed, short-hafted stabbing assegais, which forced them to fight at close quarters. He then instituted a regimental system based on age groups, quartered at separate kraals (villages) and distinguished by uniform markings on shields, headdress and ornaments. And he developed standard tactics, which the Zulu used in every battle thereafter. Each impi was divided into four groups. The strongest, termed the "chest," closed with the enemy to pin him down, while two "horns" raced out to encircle and attack the foe from behind. A reserve, known as the "loins," was seated nearby, with its back to the battle so as not to become unduly excited, and could be sent to reinforce any part of the ring if the enemy threatened to break out. Besides their prowess in battle, the Zulu warriors could cover tremendous distances, an impi consistently covered 50 miles a day, living off grain and cattle requisitioned from the kraals it passed and accompanied by young boys who carried the warriors' sleeping mats and cooking pots.