The Battle of Gettysburg as covered by The Saturday Evening Post

(CBS News) Monday marks the 150th anniversary of the Battle of Gettysburg. It was fought from July 1-3, 1863, and is considered one of the costliest military engagements in American history, in part because those on both sides were Americans.

Being the America's oldest magazine still in publication, The Saturday Evening Post has for almost 300 years covered just about every topic, including the Battle of Gettysburg. The magazine has been around since the days of the original 13 colonies.

Jeff Nilsson, historian and the Post's director of archives, said on "CBS This Morning: Saturday" that the publication had a huge advantage in covering the war because the battlefield was basically in its backyard.

The publication was based in Philadelphia, which is about 120 miles from Gettysburg, Pa. However, in the 1860s it was still quite a far distance to travel, and it did not arrive until July 3, 1863, the final day of the battle.

"They didn't have a great number of correspondents out there in the field, but they did manage to get somebody out there on the third, who reported from the battlefield just after the rebels had left and said, 'Looks like that we had won,' and covered most of the fighting, but very short, very brief notes towards it," said Nilsson. "Most of the post's coverage dealt more with how are we going to get a militia together and who's going to go into this draft and who's going to buy their way out of it."

Gettysburg has personal connection for CBS correspondent

The three-day battle is considered the turning point in the war, and it is estimated that between 46,000 and 51,000 people were killed at Gettysburg. After the battle, it is believed that the South never had a chance to win the war. They had lost too many men and equipment, and their troops were already stretched too thin.

"We know it's decisive today, but the interesting thing is that, even at the time, the soldiers marching into that were calling it decisive, were writing home and saying, 'This is the decisive battle, this is going to determine it.' The outcome of this battle would mean either the Confederacy could win and there would be two nations or the North might win and then maybe prolong the war," said Nilsson. "But they knew that there was never going to be a battle again like this, and there was never a battle of this size."

For Jeff Nilsson's full interview, watch the video in the player above.