Gettysburg and Vicksburg. They’re oft touted by historians as the decisive battles that totally won the war for the Union.
Gettysburg was a titanic clash on Northern ground but it did not neutralize the Army of Northern Virginia as a threat. Lee's army was badly depleted by the casualties but remained active and potent for another year. Everyone seems to forget that in 1864 the Army of Northern Virginia received as many as 25,500 replacements, bringing the number of men serving in Lee's army at one point or another up to 98,000 men. There also seems to be a mistaken idea that Lee never tried offensive action against after Gettysburg. Lee's Bristoe Station offensive after Meade's advance to Rapidan River to repeat the Second Manassas onto Meade is ignored. After the detachment of Longstreet from the Army of Northern Virginia and the departure of XI and XII Corps from the Army of the Potomac, Lee was still determined to take the offensive and strike Meade's rear. Not to mention Jubal Early's raid after the Overland Campaign.
Vicksburg is more of a mixed bag. The impact of the loss of Vicksburg is overstated for the Confederates and understated and overstated for the Union. The paroled 30,000 Confederates went back into service for the Confederacy. At least two divisions made it back to join Bragg during the Siege of Chattanooga, many of them were not properly exchanged. The Mississippi River was only psychologically important, not commercially. For the Union, the true reward for conquering Vicksburg was the release of Grant’s Army of the Tennessee. But the Union squandered their victory by misuse of the troops that had been tied up by the Siege of Vicksburg. The Army of the Tennessee was broken up to support Banks (XIII Corps) and the Union forces in Arkansas. The rest of the army spent the rest of summer hunting guerillas, instead of following Grant’s suggestion to begin a campaign for Mobile.