Trivia: Virtually all of the extras who play soldiers in the film were members of Civil War Re-enactor groups who not only freely donated their time but also supplied their own uniforms and equipment. This ensued that the film-makers did not have pay for their own extras and that they could stage the massive battle scenes without blowing the budget.
Trivia: During Pickett's charge, Armistead's brigade gets caught at a wooden fence. There is a close up of one of his aides rallying the men and getting shot. If you look closely you will see it's Ted Turner, who bankrolled the movie.
Trivia: Ted Turner's brief appearance is not the only notable cameo in the film. During the artillery bombardment on the Union positions prior to Pickett's charge, Union General Hancock is seen calmly riding through the barrage. A young officer begs him to come down and take cover. The officer is played by Ken Burns, the filmmaker who produced the acclaimed documentary series The Civil War.
Answer: In short, the Federal forces (who fought for the North) outlasted the Confederate army. Since it was a 3 day battle, a full explanation can not really be given here, but Lee did not actually have an accurate understanding of the Union's strength and position. While successful on Day 1, by the start of Day 2 more Federal forces arrived and the Union army had taken defensive position on the high ground. On Day 3, there was mixed communications with Confederate commanders and they did not attack as Lee had planned. The Union army was supplied with fresh forces that allowed them to hold the line. On Day 4, Lee did not attack and formed a defensive line, waiting for Meade to attack, so the Confederates could do what the Union had just done to them. But Meade never attacked and that night Lee and the Confederate troops left.
Bishop73