Factual error: During the engagement of the 20th Maine in defense of Little Round Top there is a tremendous amount of firing going on as they repel charge after charge of Confederates. Yet, there is not a single leaf, or tree branch, that falls to the ground. With that amount of lead flying around there would have been debris everywhere. There are actual Civil War accounts where whole trees were cut in half by bullet fire.
Factual error: When General Buford looks back toward Gettysburg from the cupola of the Lutheran Seminary building to see if the First Corps is approaching, the director used a colorized photograph of the town taken by Matthew Brady a few weeks after the battle to illustrate Buford's view. Unfortunately, the photo was taken from a point north of the Chambersburg Pike and the Lutheran Seminary is south of it.
Factual error: There are several scenes which show an American flag with way too many stars for that time period.
Factual error: There is one minor anachronism in the scene in which several Confederate officers are discussing politics with the British officer, Freemantle, while playing cards near a campfire. General Pickett is seen holding some paper money in one hand. The only note visible is a Confederate twenty dollar bill, with the back of the note exposed. While the note appears to be genuine, the back design is unique to Confederate twenty dollar bills issued after February 17, 1864 - more than seven months after the battle.
Factual error: As Buford's Cavalry is approaching Gettysburg there is a scene filmed from behind as Buford stops by a fence and peers out over some fields. One of the fields appears to be a corn field and the corn appears to be about 5 to 6 feet high, much too high for June 30th in that part of the country. Having been raised on a farm in New Jersey corn that was "knee high on the 4th of July" was considered a bumper crop. The height of the corn was much higher than "knee high" on what was suppose to be June 30.
Factual error: Pickett's charge moved from west to east and was made about 2 o'clock in the afternoon. When Pickett rides down the line to encourage his men before the attack, it is clear from the shadows that the scene was filmed early in the morning.
Factual error: During the scene when Gen. Lee is talking to Moxley Sorrel in the cabin AFTER little round top, he calls Moxley "major", and moxley is wearing the insignia of a Confederate lieutenant-Colonel. By that date in the war he was a lieutenant-colonel, but a couple times Lee calls him major.
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.