Well, after our Civil War ended, many people retained a cultural loyalty to the side that your country backed. Since one of that side's main causes was the continuation of the slave system that began when your country was in charge, the flag of that side became a popular symbol for racist causes in addition to whatever else people thought it meant. That usage has come to a head recently after a young man murdered several black people in what he claimed was an attempt to restart the Civil War.
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