Abolition of slavery as a war aim of the Union really didn’t come until ‘63. By that time, it was clear that, whatever else one’s sentiments on slavery, destroying the enemy’s chief labor source simply made strategic sense.
In Garibaldi’s mind, victory over the Southern slaveholders would come swiftly; “the enemy is weakened by his vices and disarmed by his conscience,” he told his comrades. From there they would go on to vanquish the slaveholders of the Caribbean and Brazil, where millions of “miserable slaves will lift their heads and be free citizens.”...
It was much more than that, for Garibaldi’s question anticipated a fundamental problem the Union confronted in trying to explain its cause to a puzzled world. Was this only a civil war, a purely domestic conflict in a quarrelsome democracy? Was the Union’s goal nothing more than to put down rebellion and protect its sovereignty? Or was there something of real consequence to the world at large? The Union would have to find answers before other powers of the world decided to include the South among the family of nations.
Secession and the Confederacy were entirely about slavery. Read the secession statements if you doubt this. However, for Unionists, the war was initially about preserving the Union. US opinion was very divided on slavery and race.
Although the case for abolishing slavery had been made since the founding it was still not clear that Washington had the authority to do so under the Constitution. Until the war.
Arguing about whether slavery or states’ rights caused the Civil War is like arguing whether it’s the fall that kills you or the sudden stop.