Simplify the following text for a Grade 7 native English-speaking person.
Original: As the Union armies advanced through the Confederacy , thousands of slaves were freed each day until nearly all ( approximately 4 million , according to the 1860 census ) were freed by July 1865 .
Simplified: Only a small number of slaves already behind Union lines were immediately freed . As Union forces advanced , nearly all four million slaves were effectively freed .