Final answer:
The most powerful tribe in the middle Missouri River that the expedition team came close to fighting with was the Lakota Sioux tribe, led by influential leaders such as Red Cloud and Crazy Horse. The Battle of Little Bighorn was a significant encounter where the U.S. Army suffered a major defeat against the Native American forces.
Step-by-step explanation:
The most powerful tribe in the middle Missouri River that the expedition team came close to fighting with was the Lakota Sioux tribe. Led by influential leaders such as Red Cloud and Crazy Horse, the Sioux were skilled in high-speed mounted warfare, making them a formidable force to reckon with. The expedition team, led by Colonel George Custer, encountered a large group of Lakotas, Cheyennes, and Arapahos at the Battle of Little Bighorn, where they suffered a significant loss in the face of Native American opposition.