Conversation so far:
when was the expansion?
began to expand on the plains.
what expanded?
the Arapaho.
what was special about the expansion?
through trade, warfare, and alliances with other plains tribes.
who did they trade with?
the Arapaho made an alliance with the Cheyenne .

Question: who did they go to war with?
Self-contained rewrite: who did Arapaho go to war with?