Robert E. Lee was a slaveholder who led an armed rebellion against the United States to keep people enslaved.
By praising him, President Trump is making a direct appeal to the politics of resentment.
This is the kind of revisionist history that has kept Jim Crow alive in the South for more than 150 years.
There is no reason to praise a Confederate general at a political rally other than to fan the flames of hate.
For more information about the Lost Cause mythology, read SPLC's report on the 1,740 Confederate monuments, place names and other symbols still in public spaces.