“The Confederate states seceded so they could preserve slavery; the rest of the United States fought to preserve the Union.”
IF what you have written is true, then we can forever dismiss the notion that Lincoln and the North fought for some high moral cause like “freeing the slaves.”
But the North did fight - and for a very important cause: what they considered their economic and political best interests.
In that context it is worth noting that the U.S. constitution before and during the war enshrined slavery and that Lincoln twice swore an oath to defend the pro-slavery constitution of the U.S.