Why did they call it the civil war?
A "civil" war is an armed conflict fought within a country, between opposing factions. The United States Civil war was between North and South. The English Civil war of the 1640s involved those loyal to King Charles the first versus a Puritan house of Commons. Many other nations have had civil wars.
If you mean the US Civil War, that’s a great question. A civil war takes place amongst the citizens of one nation. In reality, 11 States seceded, or left, the USA to form the Confederate States of America (CSA). During the war it was called the War Between the (Union & Confederate) States.