How did the war to preserve the union become a war about black freedom?

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...