The Civil War (1861–1865) was fought over the issue of slavery. But Abraham Lincoln was cautious about publicizing his strong opposition to slavery.
One reason for Lincoln's carefulness was he believed there were Unionists in the South, and he did not want to alienate them. In fact, there were few Unionists outside western Virginia.
Another very important reason for Lincoln's caution was the status of the border states (Missouri, Kentucky, Delaware, and Maryland). If Lincoln had proclaimed a war against slavery at the onset of the war, those four states might have joined the Confederacy. If that had happened, the North's position would have been untenable. When the Emancipation Proclamation (1862) was made in the second year of the war, it did not apply to the border states.
Slavery was the primary reason for the war, so the issue was never far from the surface. The South had left the Union because it feared Lincoln's plans for slavery. Also, the North did not want England and France to intervene on the South's side. Because the Anglo-French were against slavery, it was important for the North to publicly proclaim its opposition to it. After the Emancipation Proclamation, the Anglo-French had no interest in helping the South win.
The Civil War was all about freedom for black people. After the South surrendered in 1865, freedom was won.
There are three parts to the process of a war of secession becoming a war about emancipation.
First, the Southern states decided to secede from the Union because of slavery. They rightly discerned that it was only a matter of time before the system was abolished if they stayed in the Union. The tide of history was against them; the might of the industrial North was against them. The British had long since ceased participating in slavery. It was increasingly becoming what civilized nations did not do. The slave owners recognized that they would not be able to maintain their lucrative and exploitative way of life if they did not separate from their non-slave-owning peers and establish a backward-looking enclave.
Part two, however, is why Lincoln fought the South's secession. Although he wished to end slavery, the rationale for the war was preserving the Union. In other words, he would have been quite ready at the start of the war to welcome the rebelling states back into the Union as slave states if they would have been willing to come. He did not want a war. His top priority was keeping the United States intact; he could work toward abolition later.
Part three came about as Lincoln realized that the war would drag on until the South had breathed its last breath. They were not going to come back at the wave of an olive branch. Therefore, Lincoln hoped to accelerate the speed of their defeat. He believed that if he freed the slaves in Southern states and encouraged them to rise up and join the Union war effort, this would hasten the end of the war. Therefore, he issued the Emancipation Proclamation in 1863.
At this point, it was very clear that the war had moved beyond preserving the Union and into both preserving the Union and freeing the slaves. Although abolishing slavery was not realized until after the war, from 1863 onward, it was obvious that there would be no going back to slavery for the Southern states once they lost.
The war US Civil War became a war for black freedom mainly through Abraham Lincoln's Emancipation Proclamation, which he put forth as an executive order on January 1, 1863. The proclamation changed the status of all enslaved blacks in the Confederate South from slave to free. Lincoln issued the proclamation under presidential war powers in a deliberate attempt to broaden the scope of the war, making the abolition of slavery one of its goals. Any black who managed to make their way to the North or who was residing in territories that the Union Army occupied became a free man. This prompted many slaves to flee the South, escape to the North, and join the Union Army.
The proclamation outraged the Confederates, who were vehemently opposed to the liberation of their slaves. However, it turned international opinion toward the Union, which shattered Confederate hopes for allies from abroad. After the war, Lincoln followed up on the Emancipation Proclamation by promoting passage of the 13th Amendment, which banned slavery in the United States. Congress passed this amendment on January 31, 1865, and it received state ratification, finally putting an end to legally sanctioned slavery throughout the country, on December 6, 1865.