The Union's objectives in the civil war included

The Civil War began as a purely military effort with limited political objectives ... The Union war effort expanded to include not only reunification, but also the abolition of slavery. To achieve emancipation, the Union had to invade the South, defeat the Confederate armies, and occupy the Southern territory.

It began as an armed rebellion of southern states against the Union. The southern states wanted to break off and form their own country where slavery could be preserved. The Union objective was to end the rebellion and preserve the Union. Later in the War, the Union cause took on another objective: to end slavery.