In doing a bit of digging into the history of Custer I found he was apparently not generally highly regarded by the U.S. military. For instance "President Ulysses S. Grant, criticized Custer's actions in the battle of the Little Bighorn. Quoted in the New York Herald on September 2, 1876, Grant said, "I regard Custer's Massacre as a sacrifice of troops, brought on by Custer himself, that was wholly unnecessary – wholly unnecessary."""His mythologized status in American history was partly established through the energetic lobbying of his wife Elizabeth Bacon "Libbie" Custer throughout her long widowhood which spanned six decades" (both previous quotes from https://en.wikipedia.org/wiki/George_Armstrong_Custer). Without Libbie’s sixty years of campaigning, he probably would have gone down in history as a flamboyant, yet mediocre military leader rather than some sort of martyr.

The history leading up to the battle is almost impossible to distill down, but here is the gist: The battle took place on the Crow Indian Reservation. Due to the Black Hills gold rush, (technically, though inadvertently, caused by Custer), the Lakota were being displaced off their reservation in the Black Hills. The government was shrinking their reservations (and breaking their treaties) to make way for white miners. The displaced Lakota were steadily moving into the Crow Reservation. The Crow asked the U.S. Government to help remove them. At the same time, the Commissioner of Indian Affairs had issued an ultimatum for all Northern Plains Indians (Lakota, Northern Cheyenne and a small number of Arapaho) to report to their respective reservations or be labeled “hostile.” When almost no Lakota showed up, it was turned over to the U.S. Military. Custer was under orders to find and return the Lakota to the Sioux Reservation. The bands of Lakota, Cheyenne and Arapaho came together under the Lakota leader, Sitting Bull. They were gathered in a large camp along the Little Bighorn River. The U.S. Army knew the general location and sent the 7th Cavalry to “flush” them north. What was not known was how large the assembled tribal group was. When Crow and Arikara scouts for the 7th Cavalry (who also died on the battlefield) discovered the camp location, Custer moved his troops in that direction and fatefully attacked. The death of Custer and his troops became a rallying point for the United States to increase their efforts to force native peoples onto reservation lands.(https://www.britannica.com/event/Battle-of-the-Little-Bighorn, https://www.nps.gov/libi/learn/historyculture/battle-story.htm, https://en.wikipedia.org/wiki/Battle_of_the_Little_Bighorn, https://www.ushistory.org/us/40b.asp)