The US Democratic Party used to be the home of the slavers. The Ku Klux Klan was basically the armed wing of the Democratic Party. Abraham Lincoln was a Republican. History is taught and propagated by leftists institutions, so these uncomfortable facts (which often surprise leftists) are downplayed to the point of being little known.
The US Democrat Party changed all that in the 1960s, when it switched from anti-back racism to, increasingly, anti-white racism. Now it seems like the Black Lives Matter movement is increasingly like the armed wing of the modern Democratic Party, a political ally that sometimes turning up to beat up its enemies. BLM engages in political violence, such as murdering cops, so the above meme has a grain of truth to it.
Some history the PC crowd don’t want you to know:
hat-tip Scott of the Pacific