Complex sentence: Under the US Federal policy of Native American self-determination, Mankiller was able to improve federal-tribal negotiations, paving the way for today's Government-to-Government relationship the Cherokee Nation has with the US Federal Government.
Split into simpler sentences: Under the US Federal policy of Native American self-determination, Mankiller improved federal-tribal negotiations. She helped prepare for today's Government-to-Government relationship which the Cherokee Nation has with the US Federal Government.