It is hard to overstate the significance of the Civil War—in fact, one of your challenges will be conveying to your students just how important it was. You might begin by translating some statistics. Six-hundred thousand were killed, 2% of the country's entire population—that would be six million people today—and all of them were Americans killed fighting other Americans.

Or you might begin by asking your students to think about the huge importance of a simple word change—the change from the United States are to the United States is; that is, a shift from thinking of "United States" as plural to thinking of it as singular.  The Civil War made us one country.