Who is Marse Robert?
"Marse" was an term of affection and endearment used in the 1860s. The best modern equivalent that comes to mind is how a child might call an older friend of the family Uncle So-and-so even though they're not actually related. It was a very common nickname for General Lee.
Marse Robert, Uncle Robert, Marble man. Loved by the people of the South and adopted by the people of the North.
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Master Robert E. Lee.
"Four Years under Marse Robert" by Robert Stiles, 1836-1905.