Conversation so far:
What was the earliest record of the Arapaho?
Around 3,000 years ago, the ancestral Arapaho-speaking people (Heeteinono'eino' ) lived in the western Great Lakes region along the Red River Valley.
Who was the chief of the tribe?
CANNOTANSWER.
How many Arapaho were there?
CANNOTANSWER.
What can you tell me about the tribe?
The ancestors of the Arapaho people entered the Great Plains from the western Great Lakes region sometime before 1700.

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