Civil War

Prior to the Civil War people said, "The United States are," and after the Civil War people said, "The United States is," 

 Prior to the Civil war, the United States referred to themselves as "are" basically referring to everyone. At first it was seen as a united nation that worked together to fight through situations. And after the Civil War the people refered to themselves as "is" because they were much of a united nation anymore. Everyone began to have different ideas which cause them to seperate.