Conversation so far:
What was the earliest record of the Arapaho?
Around 3,000 years ago, the ancestral Arapaho-speaking people lived in the western Great Lakes region along the Red River Valley in what is classified as present-day Manitoba, Canada and Minnesota
Who was the chief of the Arapaho tribe?
Chief Niwot (c. 1825–1864), led a band in Northern Colorado and died from wounds sustained during the Sand Creek Massacre.
How many Arapaho were there?
Total population 10,861
What can you tell me about 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

Question: What made them enter a new territory?
Self-contained rewrite: What made the Arapaho enter a new territory?