Conversation so far:
When was the expansion for the Arapaho?
Once established, the Arapaho began to expand on the plains through trade, warfare, and alliances with other plains tribes. Around 1811, the Arapaho made an alliance with the Cheyenne
What expanded when the Arapaho began to explan on the plains?
Their strong alliance with the Cheyenne allowed the Arapaho to greatly expand their hunting territory
What was special about the Arapaho's expansion on the plains?
Conflict with the allied Comanche and Kiowa ended in 1840 when the two large tribes made peace with the Arapaho and Southern Cheyenne and became their allies.

Question: who did they trade with?
Self-contained rewrite: who did the Arapaho trade with?