Final answer:
The Indian leader who first spoke out against the American advance upon the plains is Chief Sitting Bull. He led his people with strong resistance to U.S. policies and became known for his crucial role in defeating Custer at the Battle of Little Bighorn.
Step-by-step explanation:
Chief Sitting Bull was a Hunkpapa Lakota Sioux spiritual leader renowned for his staunch resistance to the United States government policies and his leadership role in the famous Battle of Little Bighorn.
True to his name, which means the strength of an intractable bison, he united multiple tribes to resist forced removal through a variety of methods, including armed resistance. He met his end when he was killed by authorities during an attempt to arrest him, sparking significant tensions that would lead to tragic events, such as the massacre at Wounded Knee Creek.
Sitting Bull's resistance not only exemplified the Native American struggle for land and cultural integrity during Westward Expansion but also inspired other leaders, like Crazy Horse, to fight for their rights to sovereignty and way of life. Their efforts stand as a testament to the fierce determination to protect their land and people.