Answer:
He gave as a reason to start the war the assertion that it was necessary to save the union.
Step-by-step explanation:
In Abrahm Lincoln's letter sent to Horace Greeley he states that his attitude in allowing civil war to be fought, is not to save blacks from slavery, or to raise the morale of northerners, but because that is the only way to save the union. As proof of this, we can consider the lines: "What I do about slavery, and the colored race, I do because I believe it helps to save the Union; and what I forbear, I forbear because I do not believe it would help to save the Union. "