Answer: The Union winning the Civil War
Step-by-step explanation:
President Lincoln in that excerpt is saying that if the war was to be lost then the men and women who died in the civil war defending a type of governance that was of, by and for the people and guaranteed freedom for humans will have died in vain.
Whilst there were multiple reasons for the Civil War, the most popular one was that of slavery. The Confederacy did not want to give it up and the Union wanted to abolish it.
President Lincoln as the leader of the Union is therefore stating that if the Union did not win, the freedom that they fought for would be crushed by the Confederacy and perish from the world and therefore would have been in vain.