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The 400 yr nonsense is the same dims saying we're a democracy all the time.
They know repetition matters.
1619 first slave ship arrives in the British colonies.
1776 US declares independence.
1863 Emancipation Proclamation frees slaves in the Confederacy
1863 13th Amendment Ratified ending all slavery in the US.
For 87 years slavery existed in the US. Hardly the 400 years claimed.
Edit. 13th ratified in 1865.