Slavery was an issue between the Northern states and the Southern states, which was a huge reason why the Southern states seceded from the Union and became the Confederacy. The Northern states were free states, meaning that they did not have slavery in their territories. Some people in the Northern states even wanted to abolish slavery completely. The Southern states were slave states and they depended on slavery for their income.
The balance between slave and free states had been a problem before, but the South left the Union after Abraham Lincoln's election in 1860 because they felt that it was a threat to slavery. The South created the Confederacy, which wanted to keep slavery and would fight those who threatened to take it away.
The main goal of President Abraham Lincoln was to "preserve the Union." He wanted to end the war as fast as possible and become a whole country again, he felt like that was his duty. So at first, the Civil War was not really about freeing the slaves whatsoever, but "preserving the Union."
The goals of the Civil War shifted after the Emancipation Proclamation, which was issued by President Abraham Lincoln on January 1, 1863. The Emancipation Proclamation freed slaves only in the states that rebelled against the Union, also known as the Confederate states. This did not free the slaves in the border states. Since the slaves were now freed in the Confederate states, it caused many Southerners to be confused and fearful that they would lose their labor force. This caused the Civil War to be about slavery now, which caused Europe to not want anything to do with the South anymore because Europe did not agree with slavery.
