Which battle marked the first significant victory for the Union during the Civil War?

Chancellorsville

Bull Run

Fredericksburg

Antietam

The correct answer is

Antietam

The remaining three battles were won by the confederate forces. The battle of Antietam was almost a stalemate but is generally considered to be a strategic victory for the Union. Five days after that battle the Emancipation Proclamation was issued by the President.