Complex sentence: Upon conquering the US, the Einsatzkommandos and the American white supremacists systematically kill of most of the country's Jewish and Black populations with any remaining Black populations being used for slave labor by the Reich.
Split into simpler sentences: Upon conquering the US, the Einsatzkommandos and the American white supremacists systematically kill of most of the country's Jewish and Black populations. The remaining Jewish and Black populations are used for slave labor by the Reich.