Why did slaves join the union armies?
The Union armies were against slavery, as one of their key points to fight about, and freed slaves often would join in. Slaves could also usually remain free in the North (thus, the Union Army) and many other slaves wished to be free. If the Union were to win the Civil War, then it was highly likely that slaver would be abolished so many salevs chose to help fight in side with the Union for that reasons.