Which battle proved the most significant in the Union's eventual victory and why. Bull Run (1st Manassas) July 21,...

This is pretty much of an opinion question as there can't be any objectively provable answer.

It seems to me that you could argue for many of these:

• The Union victory at Vicksburg returned full use of the Mississippi to the Union and denied it to the Confederacy.  This greatly affected the abilities of the two sides to move freely North-South.


• The victory at Gettysburg would mark the last time that the South was really on the offensive.  You can argue that an army fighting solely on the defensive is doomed and, therefore, you can argue that this doomed the Confederacy.


• The victory at Chattanooga opened up the way to the Deep South and allowed Sherman to take Atlanta and then march to the sea.  It also allowed Grant to be taken from the West and moved to fight directly against Lee.

If I had to pick, I'd probably pick Chattanooga because the Sherman's action and the presence of Grant to fight against Lee were so important.