Why was the Union army defeated at Chancellorsville

Because Joseph was too scared to take the risk when battling the confederates.

During the battle of chancellors vile, General Lee brought a way smaller number to the battle compared to joseph's.

This made joseph hookker became hesitant, thinking that Lee made a lot of traps. Because of this, joseph's missed an opportunity to finished the confederate in one swoop and prolonged the fight necessarily. In the end, Union's casualties were larger than the confederates.