Pathos

While Lincoln had the stature (dude was tall) and gravitas that comes with being president, he was hardly the main attraction of the ceremony. He was there to give some short remarks following Edward Everett, the preeminent speaker of his time.

But the president's heartfelt appeal to a tired and reeling populace (along with some important military victories) turned the tide of the war in favor of the Union.

Get it, Abe.

Consider this line:

The brave men, living and dead who struggled here have consecrated it far above our poor power to add or detract. (7)

If he wasn't nicknamed Honest Abe, it might seem to be a bit of pandering. But seriously—every person in the United States was touched by the Civil War. And by reflecting on the sacrifices made by soldiers (both living and dead), Lincoln was guaranteed to have the support of those listening.

War speeches aren't about logic, they're about fanning the flames of passion. And the Gettysburg Address was like a bellows strengthening Northern resolve.