Obviously, the South was not totally unified in political thought and purpose - any nation that goes to war rarely is. Southerners fought for different reasons: defense of home, slavery, protection of family, states rights, etc, etc. And there were pockets of pro-Union sentiment in the South as well, as Robert has pointed out.
And while there are others who like to remind every one that the South was not monolithic in their views on the struggle that embroiled our nation between 1861 and 1865, these same individuals are quick to do a 180 and say the South was monolithic in "fighting for slavery". Can they have it both ways? No, but they try.
With all this in mind, I'd like to remind readers that there was also quite a bit of pro-South sentiment in the North. I'll mention one example in this post and some more later. . .
Over the years, the people within New Jersey had developed many family ties with the South, and several leading Confederates were born in New Jersey. Among them were Henry Ellet, who was offered but declined the position of Postmaster General for the Confederacy; Samuel Cooper, Adjutant and Inspector General of the Confederate Army . . .
The census statistics of 1860 indicate that 6,068 Southern-born New Jersey inhabitants had moved to New Jersey. This was about one percent of the State's total population. Even more significant was the movement of Jerseymen to the South. Of the 16 Southern states, New Jersey supplied more inhabitants in seven than any of the three major midwestern states (Illinois, Indiana, and Ohio). . . There was, indeed, a definite north-south movement between New Jersey and the South.
These economic and social ties clearly mark New Jersey as a border state. Its attitude towards slavery and its political position during the secession crisis confirm this status. . .
Two [New Jersey] newspapers advocated that New Jersey unite with the South. There were the Newark Journal and the New Brunswick Times.
Three newspapers wanted New Jersey to secede from the Union and join the other border states to form a central confederacy. . . On December 27, 1860, the Monmouth Democrat printed an editorial in which it favored the central confederacy as it was "better than to risk the results in a civil war." This same editorial was printed in the Hunterdon Democrat of January 2, 1861. As an introduction, Hunterdon's editor wrote:
"Peaceful Secession - We agree with the following views taken from the Monomouth Democrat, in regard to Peaceful Secession."
This editorial stated:
"We are in favor of peaceful dissolution, and opposed to all measures of coercion. If the Union cannot be preserved without shedding the blood of our brethren, it cannot be preserved at all."
The above excerpt is from: The Secession Movement in the Mid-Atlantic States by William C. Wright (Associated University Presses, Inc., 1973), pages 98-99 & 113.
There were diverse opinions in the South, of course. But the same is true of the North. To suggest that the North's sentiment was uniformly "pro-Union" is as inaccurate as suggesting that the South was uniformly "pro-Secession."