Describe how the war to save the Union became the war the end slavery

There was also another factor that led towards a shift of
focus: military realities.  By 1863, the Union had lost hundreds of thousands of men. 
In July of 1863, the New York Draft Riots broke out after long lists of the dead of
Gettysburg were posted next to new lists of draftees.

In
other words, the Union had both a motivation and a potential manpower issue.  By
changing the focus of the war towards ending slavery, Lincoln won the unequivocal
support of northern abolitionist churches, the abolitionist movement, and northern free
blacks as well.  180,000 African-Americans joined to fight against the South, in much
higher percentages than their actual portion of the population, and were some of the
most highly motivated, hard fighting soldiers of the
war.

Also by that time, it was getting harder to sell the
idea to the American public that the war was about restoring the Union alone, as many
were starting to believe it was not worth the price.