Lincoln's speech at Gettysburg, PA, in November, 1863--widely considered to be the address that help to heal divisions within the country--has two main and interconnected thesis statements in this very persuasive speech.
His first thesis centers on his and his listener's inability to dedicate, in any way other than technical, the cemetery:
. . . we cannot dedicate, we cannot consecrate--we cannot hallow--this ground. The brave men, living and dead . . . have consecrated it far above our poor power to add or detract.
Lincoln's "argument" here is that no one except the combatants can effectively "hallow" this ground because it is only they who have given of themselves to insure the continued existence of the United States. Lincoln is arguing here that although we can honor the soldiers for their sacrifice we have no right to "consecrate" the land over which they fought and died because the cost was theirs not ours.
And the second, but very closely aligned argument, is that although we--the onlookers--can not consecrate the land already consecrated by our soldier's blood and devotion we can, and must,
. . . resolve that these dead shall not have died in vain--that this nation, under God, shall a new birth of freedom. . . .
In essence, Lincoln's theses are that, one, we do not have the power (actually, the right) to consecrate this hallowed ground, but, two, we do have the obligation to insure that our soldiers, who fought to keep us one country, succeed in the goal for which "they here gave the last full measure of devotion."