What was the most significant contribution of the Spaniards to the Native Americans?

The introduction of the horse by Spaniards to Native Americans was indeed a pivotal moment in their history and culture. Horses transformed the way many tribes hunted, traveled, and conducted warfare. The ability to ride horses allowed for greater mobility and the ability to cover larger distances, which was particularly significant for tribes in the plains region who relied on bison hunting as a primary food source. The horse also changed social structures within various tribes, enabling some to become more formidable in conflict and enhancing trade networks due to improved transportation of goods.

While gunpowder, religion, and iron tools did have their impacts, the horse's contribution was fundamentally transformative in ways that were deeply ingrained into the daily lives, economies, and cultures of Native American societies after its introduction.