Brother against brother, family against family
The Civil War was a war fought by Americans against other Americans. Regardless of which side people chose, they were all Americans.
The people in America ranged from families living in the United States for many years, to recent immigrants who had come to a New World seeking opportunities, or freed African-Americans who fought to end slavery. Over the years, their beliefs had divided the country into North and South. These Americans had shared a common history, but it was their view of the future that divided them.
The Civil War was called a "war against brothers" because the United States was like a family, and although a person might not get along with his or her siblings, they were still a family. Whether from Pennsylvania or New York, South Carolina or Georgia, families in the North most likely knew of families in the South who fought against them. Sometimes, members of the same family went off to fight for different sides.
Some even lost their friends to the other side; the other cause. Families in both the North and South struggled to survive during this bitter war. Friends, relatives, and family members would sometimes meet on the battlefield and be forced to fight one another. It was truly a war of brother vs. brother.