Short answers, pulled from the story.
The Union army's victory at the Battle of Gettysburg, fought July 1-3, 1863, combined with the Union capture of Vicksburg on the 4th of July 1863, is most frequently cited as the decisive turning point. Together, these events ended Lee's offensive ambitions and split the Confederacy in two along the Mississippi River.
Lincoln's cabinet advised him to issue the Emancipation Proclamation only after a Union battlefield victory, to avoid it appearing as an act of desperation. The Battle of Antietam on the 17th of September 1862, provided that victory, and Lincoln announced a preliminary proclamation shortly after, stating that on the 1st of January 1863, all enslaved people in rebellious states would be declared free.
The fall of Vicksburg on the 4th of July 1863, split the Confederacy along the Mississippi River, cutting off supplies from Texas and Arkansas. Lincoln had described Vicksburg as the key to ending the war, and European bond investors dropped their estimated probability of a Confederate victory from roughly 42 percent to about 15 percent by the end of 1863 following its fall.
On the 3rd of September 1861, Polk ordered the occupation of Columbus, Kentucky, without authorization from the Confederate government, violating the state's declared neutrality. This enraged Kentucky's citizens and legislature, turning the state against the Confederacy and denying Confederate forces a safe area of operations in a strategically vital region controlling the Tennessee and Ohio rivers.
Jackson died of pneumonia following a friendly fire wound after the Battle of Chancellorsville in May 1863, two months before Gettysburg. Many historians argue that Jackson might have seized key positions including Culp's Hill and Cemetery Hill on the first day of Gettysburg, positions his replacements were unable or unwilling to take. Lee himself told subordinates they should have acted as Jackson would have.
Grant was appointed general-in-chief on the 12th of March 1864, and devised a coordinated strategy striking at the Confederacy from multiple directions simultaneously, including against Lee and Richmond, the Shenandoah Valley, Atlanta, Confederate rail lines, and the port of Mobile. Unlike previous Eastern Theater commanders, Grant pressed forward even after tactical reverses, maintaining unrelenting pressure on Confederate forces until the surrender at Appomattox Court House.