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

Question: what was special about the expansion?
Self-contained rewrite: What was special about the Arapaho's expansion on the plains?