Conversation so far:
what is warfare?
Three factors led to a growing importance of warfare in Plains Indian culture.
what was the first factor?
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?
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 in warfare?