This thread might be of interest:
Why preserve the Union?: Unionism vs Secessionism
From the first (my) post in the thread:
Slavery explains the reasons for disunion, it does not explain the reasons for union. That is, slavery does not explain the reasons why so many Americans wanted to keep the Union together.
These two forces - secessionism and unionism - were kind of like a yin and yang. You don't get the full picture until you see both sides of the circle. Protecting slavery was the driving force for the white southern elites who championed a separate slave states' nation.
...unionism, IMO, was driven by the feeling that the putative Confederate regime was an economic, military and geo-political threat that was created by traitors who sought to annul an election they lost fair and square. And the threat being made, would not be ignored.
...As I posit above, there was a force called unionism that compelled Americans to preserve the Union in the face of slave state secessionism. Unionism was not northernism: there were southerners who supported preserving the Union.
My opinion is that 1860-1865 unionism was mainly a response to the actions of secessionists. Unionists did not care so much about the reasons that caused the slave states to dissolve the union (although many recognized that slavery was the underlying cause); they were upset that the union was being dissolved, period. Even more, the method that the union was dissolved - by armed force - was especially outrageous. This opinion may be controversial.
Unionism was the cause of the United States resisting the political anarchy and military and economic threat that was the Confederacy. Now, it that why an individual soldier fought? Maybe, maybe not. Many fought out of Unionism, for sure. Many fought out of patriotism - and that's NOT the same as Unionism. Some fought because the bonus payments were a great inducement. Some were drafted. Some were abolitionists who did want to see the slave power destroyed, but this was surely a minority.
- Alan