Go to the aid of the Army of the Cumberland at Chattanooga.
The Union could control the Mississippi River from Vicksburg
Gettysburg, Vicksburg, the Battle of Chattenooga, and the Battle of Pettersburg. You could argue that that the battle of Antietam was also a key victory as it gave Abraham Lincoln the grounds to issue his preliminary emancipation proclamation, but Antietam was not a stunning victory for the North.
The Battle of Big Black River Bridge was fought on May 17, 1863 in Mississippi. It was an important Union victory in that the Confederate forces were forced to leave the field and take refuge in the City of Vicksburg. No Southern army could rescue Vicksburg and Union General US Grant began a siege on Vicksburg which would cause Vicksburg to surrender on July 4, 1863.
There was no captain named Vicksburg in the civil war, so no person of that name could have been important to the union or anybody else. Perhaps you are thinking of the battle of Vicksburg, which was critical.
It could be either of Gettysburg or Vicksburg - perhaps the very fact that they happened on the same day, and that Confederate troops could not have been sent from one war-zone to another very distant one, to relieve the situation at either end.
You could say Gettysburg - remembering that Vicksburg happened on the same day (July 3rd 1863). But there was always the prospect of Lincoln losing the Presidential Election of November 1864. From that angle, you could say it was the Battle of Atlanta (August-Sept. 1864), although ironically this was not a victory, according to US Grant, who had ordered Sherman to destroy the Army of Tennessee - which had got safely out of Atlanta.
You could argue that there were two major battles in the war. The battle of Vicksburg was the major turning point. The battle of New Orleans sealed the deal for the North.
No, the Battle of Dien Bien Phu was not actually a victory for the French and so could not be considered the only French victory over the Viet Minh. About 40,000 Viet Minh attacked 15,000 French soldiers during this battle.
The Southern defeat at Gettysburg is regarded as the pivotal moment of the war, after which the Confederates started to look like losers. But the surrender of Vicksburg (on the same day as Gettysburg) and the earlier Union victory at Shiloh could both qualify as key events in the conflict.
The fall of Vicksburg was the victory. The Union forces from the South were able to connect with the forces from the North and divide the Confederacy in two. They could no longer count on supplies from the western states.
1 - Lee's Army of Northern Virginia could no more engage themselves in large scale invasion of Union States and become more and more pinned down on the defensive. 2 - The Confederacy had to give up every hope to be acknowledged as an Independent State and being supported by Great Britain and France. 3 - The victory added to that of Vicksburg raised again the moral of Union's home front.
The Union's forces ate horses, dogs, and anything else that they could find, including mice.