GETTYSBURG, Pa. — In solemnity, thousands gathered at a central Pennsylvania battlefield park Tuesday to honor a speech given 150 years ago that President Abraham Lincoln predicted would not be long remembered.

The inspirational and famously short Gettysburg Address was praised for reinvigorating national ideals of freedom, liberty and justice amid a Civil War that had torn the country into pieces.

“President Lincoln sought to heal a nation’s wounds by defining what a nation should be,” said Pennsylvania Gov. Tom Corbett, calling Lincoln’s words superb, his faith deep and his genius profound. “Lincoln wrote his words on paper, but he also inscribed them in our hearts.”

Echoing Lincoln, keynote speaker and Civil War historian James McPherson said the president took the dais in November 1863 at a time when it looked like the nation “might indeed perish from the earth.”...