From Slave to Soldier — Deborah Hopkinson

Johnny loves his Uncle Silas and his master's mule, Nell, but he hates being a slave. So when a group of soldiers marches by the farm and asks Johnny if he wants to join the Union Army, Johnny accepts, even though it means leaving Uncle Silas and Nell.
Being a soldier isn't easy. Johnny is given a chance to work with a soldier named Dave and the company's mule team, but he still wonders if he has made the right choice.
Then one night a hungry patrol is out on watch across the river from the camp, with no one to bring them food. Can Johnny handle the mule team all by himself?
This story was inspired by the life of John McCline, who, as a young boy, escaped slavery to serve in the Union Army.