In the July 1863 battle, considered the turning point of the war, federal forces fought back a Confederate invasion of Pennsylvania. Lincoln's speech was delivered more than four months later, at the dedication of a national cemetery to bury the battle's casualties.
In the short oration, he spoke of how democracy itself rested upon "the proposition that all men are created equal," a profound and politically risky statement for the time. Slavery and the doctrine of states' rights would not hold in the "more perfect union" of Lincoln's vision.
"In 272 words he put together what everyone was thinking, what everyone should know," said park historian John Heiser. Because of varying transcriptions, scholars generally put the text at 268 to 272 words.
Supreme Court Justice Antonin Scalia administered the oath of allegiance to a group of 16 immigrants, telling them the national identity is unique, illustrated by the existence of the word "un-American" and by the people's "fidelity to certain political principles."
Greta Myer, 44, decided to make the six-hour trip from Akron, Ohio, with her husband and son after spending a week in Gettysburg earlier in the year.
"It's something we've never done before," Myer said. "It was a historical event that we wanted to be a part of."
Among many re-enactors on the grounds were at least two Abraham Lincolns, including one who recited the address.
"Lincoln would have been surprised by the reverence accorded to him by future generations," McPherson said, noting Lincoln himself held in high regard the country's founders.
"Would they preserve that heritage, or would they allow it to perish from the earth?" McPherson said.
He said the Gettysburg Address, despite its short length, managed to weave together themes of past, present and future; continent, nation and battlefield; and birth, death and rebirth.
"Men died that the nation might live," McPherson said. "Yet the old nation also died," and with it, the system of bondage that enslaved some 4 million Americans.
Part of the event was a speech delivered by suburban Philadelphia high school junior Lauren Pyfer, who won a contest to write a contemporary version of the Gettysburg Address, but at the same short length.
She urged the crowd to do their part to "nurture and preserve the rights of humanity, equality and freedom, across all nations."
"It is impossible for one country to close its doors to other countries and still thrive," Pyfer said.
Interior Secretary Sally Jewell, who also adopted Lincolnian brevity, said the Gettysburg battle stands at the vortex of American history, and the Gettysburg Address at the vortex of national consciousness.
Lincoln, she said, called the country to its unfinished business, and he also came to symbolize the country's "greatest virtues of humility, of honesty and decency."