Final answer:
The Battle of Little Bighorn, fought on June 25, 1876, was the event considered a victory by the Plains Native Americans but marked the onset of their undoing as U.S. government forces intensified suppression efforts.
Step-by-step explanation:
The event seen as a "victory" by the Plains Native Americans in June of 1876 but later proved to be the beginning of the end was B) The Battle of Little Bighorn. On June 25, 1876, Colonel George Custer attacked what he believed to be a minor encampment, but instead faced a significant force of Lakotas, Cheyennes, and Arapahos.
The broader impact of the battle was profound as it led to a fierce response from the U.S government, which intensified efforts to suppress the Native-American resistance. Figures like Sitting Bull and his followers faced dire consequences, including being forced off their lands, killed, or fleeing in fear.