Final answer:
Sitting Bull, a Lakota Sioux spiritual leader, planned a brilliant strike against the U.S. Army in 1866, leading to their defeat at the Battle of Little Bighorn. This event is widely known as Custer's Last Stand and has been studied by historians and depicted in various forms of media.
Step-by-step explanation:
The Lakota Sioux spiritual leader who planned a brilliant strike against the U.S. Army in 1866 was Sitting Bull. Sitting Bull is most famous for his leadership in defeating George Armstrong Custer at the Battle of Little Bighorn in 1876.
Custer, leading the U.S. Army's 7th Cavalry, underestimated the size of the Native American encampment and suffered a significant loss at the hands of Sitting Bull and his allied tribes. This battle was a major event in the era of westward expansion and has been studied by historians and portrayed in movies and books as Custer's Last Stand.