While it is true that as a result of the Civil War the institution of slavery was abolished it was not he reason the war began in the first place. The primary issue was whether the southern states had a lawful right to secede.
While only 30% of the people in the United States lived in the South they were paying 80% of the taxes. Like the colonists before them they felt that they were being exploited and they decided to declare their independence and start a new nation.
The North could not survive financially without the tax revenue generated in the Southern States. In order to prevent the secession of the Confederate States, Lincoln instigated the war.
If the war was about slavery in the beginning, Lincoln could not have fielded an army to free the slaves, but he could it he suggested the cause was to preserve the union. After two years of fighting, the prospects for lincoln winning the war were not promising.
When on January 1, 1863 Lincoln issued his
Emancipation Proclamation which turned the tide of the war in his favor. This war time proclamation was a strategy to motivated slaves to enlist in the Union army and to discourage European nations from offering financial or military support to the confederacy.