Why was the Battle of Gettysburg a turning point in the American Civil War?

The Battle of Gettysburg was an important turning point because it ended with the repulse of the final major invasion of the North by Confederate forces. General Robert E. Lee brought his Army of Northern Virginia into Pennsylvania hoping to score a major victory on Union soil, a development that he thought would bring the war to an end and achieve Confederate independence. Of course, the Confederate forces were defeated at Gettysburg, and from then on, the Army of Northern Virginia was on the strategic defensive. 

Another reason the battle was such a significant turning point was that the Confederates, already stretched for manpower, suffered horrific casualties, particularly on the third and final day of the battle. Lee's army suffered over 28,000 casualties, over one-third of his pre-battle strength. These losses proved difficult to replace for the Confederacy, unlike the Union, which suffered similar losses but drew from a far superior base of manpower.

Finally, timing made this a particularly devastating blow for the Confederacy. The battle ended in Union victory on July 3, 1863. One day later, the Confederate stronghold of Vicksburg, Mississippi surrendered to a Union force under General Ulysses S. Grant. This gave the Union Army complete control over the Mississippi River, severing the Confederacy in two. While there was still much bloody fighting to do, the fact that Gettysburg and Vicksburg occurred in such rapid succession made the defeat at Gettysburg all the more devastating, both in terms of morale and strategically.