This is about Bitterly Divided, which is about the southerners, both black and white, who opposed secession (the majority of whites outside the deep south) and the civil war.
The north's advantages were considerably amplified by many southerners-- a quarter of the Union army was southerners.
Q: The deprivations at home led to a very high desertion rate among Confederates. How bad was it?
A: By 1864, two-thirds of the Army was absent with or without leave. It got worse after that.
Q: There was a sort of Underground Railroad for deserters?
A: Yes. It surprised me that many Confederate deserters could count on the support of slaves to hide them and move them from one location to another.
Q: How important were black Southerners in the outcome of the war?
A: They were very important to undermining the Confederate war effort. When slaves heard that Abraham Lincoln had been elected, many of them thought they were free and started leaving plantations. So many eventually escaped to Union lines that they forced the issue. As other historians have said, Lincoln didn’t free the slaves; the slaves freed themselves.
Link thanks to laurent_atl.