How did the Gettysburg Address change the nature and purpose of the Civil War?
The main significance of the Gettysburg Address lies in the following words:
That we here highly resolve that these dead shall not have died in vain—that this nation, under God, shall have a new birth of freedom.
In other words, the immense sacrifice of the Union dead, commemorated by this new National Cemetery, will have been in vain if the traditional understanding of American freedom remains unchanged. The Union Army is no longer simply fighting against Southern secession; it is also fighting for a radically different understanding of what constitutes freedom in the United States. With the passing of the Emancipation Proclamation, there is now a new purpose to the Civil War.
This shift of emphasis is important because Lincoln wants Americans to think carefully about what kind of country the United States will become after the war is finally over. This will be a country no longer tainted by the sin of slavery. As such, the common understanding of freedom will change forever. The tradition of freedom bequeathed by the Founding Fathers was a negative freedom, a freedom from British rule. In some respects, this was the kind of freedom invoked by the Confederacy in their act of secession.
The tradition of negative freedom had served the United States well and still had an important role to play in guarding against the excessive involvement of government in people's lives. With slavery abolished, however, it was now necessary for government to give life to the freedom of former slaves and to add substance to the formal notion of liberty upon which the United States was originally founded.
Abraham Lincoln's Gettysburg Address shifted views on the purpose of the war. Before his speech, people could have argued that the war was over states' rights vs. federal government. Or people could have argued it was a war over the issue of slavery. The Gettysburg Address changed that.
Lincoln shifted the view from those stated above to the issue of preserving the nation that was created by the founding fathers. Lincoln is calling to attention the purpose of the Declaration of Independence and all of the beliefs and values that it stood for then. Lincoln is asking the people to remember that those principles still matter. His speech is forcing people to question the validity of that government and those founding fathers. North or South, those men were respected and revered. Lincoln is suggesting that if the Union falls apart, it's equivalent to spitting on the ideas that founded the country.
The Gettysburg Address is saying that those that died there should not have died in vain if the Union is preserved. They will have died for a new birth of freedom in the United States of America.