Custer's legacy, and how opinion of it has shifted throughout time, is a fascinating subject. You are correct in saying that he was highly regarded at the time of his death, but it wasn't long before certain individuals tried to change that. However, as far as I'm aware, Custer wasn't really a victim of the Lost Cause movement. I would argue that the shift in opinion was due to three main factors:

• His personal and political enemies exploiting his failure at the Battle of the Little Bighorn to denounce him and lay blame entirely on his shoulders so that they would not have to bear any themselves, including President Grant.
• Movements to condemn the treatment of the Native Americans by the US began to use Custer as a scapegoat for all atrocities committed against Natives, despite the fact that Custer was far from the most active, vicious, or successful "Indian fighter."
• Hollywood and media depictions of Custer typically portray Custer either as either a glorious hero or a maniacal villain. Rarely is he depicted as he actually was.

It truly is a shame that so many people today only know the Hollywood version of Custer. His triumphs during the Civil War were great, so much so that at the end of the war General. Sheridan wrote to Custer's wife "there is scarcely an individual in our service who has done more to bring about this desirable result than your gallant husband."

Unfortunately, he has become a man shrouded by controversy, and even today people, be them expert historians or laymen, argue about Custer's character and competence. I could go on and on about this, but I'm on mobile and this is getting long.

Historian Robert Utley wrote a book covering this exact subject, called Custer and the Great Controversy. I recommend checking it out for further reading. For general biographies of Custer and his Last Stand, I recommend:

• Custer by Jeffrey Wert
• Cavalier in Buckskin by Robert Utley
• A Terrible Glory by James Donovan