Conversation so far:
What was Plains Indians warfare?
The earliest Spanish explorers in the 16th century did not find the Plains Indians especially warlike. Three factors led to a growing importance of warfare in Plains Indian culture.
What was the first factor that lead to warfare with the Plains Indians?
First, was the Spanish colonization of New Mexico which stimulated raids and counter-raids by Spaniards and Indians for goods and slaves.
What was the second factor that lead to warfare with the Plains Indians?
Second, was the contact of the Indians with French fur traders which increased rivalry among Indian tribes to control trade and trade routes.

Question: what was the third factor?
Self-contained rewrite: What was the third factor that lead to warfare with the Plains Indians?