In the first paragraph of the "Gettysburg Address," Lincoln mentions the two ideals on which the United States was founded. First, the new nation was dedicated to the idea of liberty, and second, to the idea that all men are created equal.
In the second paragraph, Lincoln turns to the fact that these ideals have been put to a test. A group of Southern states is waging a war to try to break up the country, and therefore break up a nation founded on liberty and equality. A battle has just been fought in which many Northern troops gave up their lives to defend these ideals. It is important to remember and dedicate a portion of this field to these men.
In the third paragraph, Lincoln develops the idea that the best way, however, to honor the dead is not to consecrate a battlefield but to keep struggling to preserve the nation. It is important to win the war to ensure that a country founded on the ideas of freedom and equality—a country based on a great and idealistic experiment—should not die.
Lincoln is trying to keep his listeners focused on the cause for which the soldiers died rather than the fact that so many young men perished at Gettysburg. He wants to unite his followers around a patriotic cause, one worth dying for.