In the 9th century, the Frankish kingdom was under attack from all sides. Vikings from the northwest and Arabs from the southeast. Martel and other Charlies discovered a new fighting technique: mounted shock troopers, also known as knights.
Rather than using a spear on horseback as before, knights would keep the spear affixed directly to their shoulder and wrap their arm around it to guide it. Then, when the horse charged and the lance tip found its target, the full force of armor, horse, and rider (1500 pounds) would be administered through the lance tip. This had a predictably devastating effect, so much so that the lance would enter its subject and remain lodged inside, effectively disarming the attacker. High backed saddles and stirrups provided the necessary stability to keep the rider on the horse at impact.
But everything about this new way of fighting was fabulously expensive. There just weren’t enough people in the ruling class to staff the armies required for this. This was one reason for the feudal system, where a vassal could swear fealty to his lord and receive lands from which he could profit, in exchange for providing military service on the lord’s behalf. This land would then provide a means of income from which the knight could supply himself and the required entourage: multiple horses, armor, squires, training, etc.
graph TD A[Invading Armies] --> |Need to be repelled| K[Knights] K --> |Require wealth| F[Lord-Vassal relations] style Inventions fill:yellow,stroke:#333,stroke-width:4px Inventions[Stirrups<br/>High backed saddles<br/>Couched lances] --> K
Source: Early Middle Ages