The answer is that they thought that it would be a short yet glourious victory.
Explanation:
The emotions among the people when the Civil War started was mixed. People were overwhlemed with the amount of killing, yet didn't want to stop it until they got their way. The Battle of Antietam is the deadliest one day battle in US history. So you can imagine how much that shook up the people back at home. Photography was also very new and for the first time their were pictures available of the battlefield. It was traumatizing, yes, but the main mind set during the Civil War was to kill as many people who didn't approve with you as you could to protect your lifestyle. (Sorry it's so long :P hope you understand now)
