The battle of Gettysburg  July 1st to July 3rd 1863 was the decisive battle of the American Civil, simply because it effectively stopped any further advance of General  Robert E Lee's confederate forces into Union territory. For that point alone it was the turning point of the war, and further it marked the end of what had been seen previously as Lees invincibility.  
It also marked a decline in Lee's  faith in his own  invincibility, possibly brought on by the heart condition that later killed him, affecting his judgement.
That judgement was wantonly lacking in his descision on the 3rd day of the battle to attack a much more heavily defended centre sector of the Union lines dug in on Cemetary ridge, rather than the left or right flanks. Had he been able to win ascendency of the left flank at 'round hill' which was only lightly defended, his artilliary would have had command of the whole Union defences. As such it was one of the military blunders of the American civil war  and helped turn the tide of the battle

Robert E Lee
Just couldnt see
His pride and his Judgement
The worse
The advance in the North
As Jackson went forth
Over the mountain wall 

To Friedrickburg town
As the Greys turned around
It marked the limit of all  
Lack of food and provisions
But plenty munnitions
Stopped the advance guard
Right in there  its tracks

So they retraced their step
A cause was all  kept
Robert E Lee damn well knew it
A battle would loom
To set the Souths doom
With out shoes on their feet
They would rue it

In Gettysburg town
A place of renown
A famous boot factory stood
They would take it by storm
And then would fight on
But the union
Had other Ideas

And so on the fields of valour at Gettysburg Pennsylvania for three days at the beginning of July 1863 Robert E Lee saw his dream of enlisting the help of Foreign governments to the confederate cause smashed.
His advance Northwards had taken the Union by surprise and since a number of Congessmen were in favour of negotiating a deal with the break away states Lee judged that if he could make further incursions North then the war might be ended  by negotiated armistice.

The fact that his provisions and supplies of clothing especially boots were virtually non existant made it inevitable he could no longer continue the Northern campaign. He realised that ultimately they would clash with a large Union force and decided to retreat before winter set in and cross the Potomac before Autumn. His retreat was  blocked at Gettysburg quite by accident

Tthe battle hence the course of history was changed essentially by bad fortune. At Gettysburg was a shoe factory and Lee calculated that if he took that he might hold up on  the  other side of the Potomac until the following spring.

Unfortunately Union generals also realised the significance of the Gettysburg shoe factory and got there first.