The Southern States honestly thought they could start, fight, and win a war against the Union that was very difficult for them to win.
Wishful thinking was common on both sides. Winfield Scott was considered a pessimist, if not an alarmist, for thinking the war would require 300,000 troops and two or three years to defeat the rebellion.
A few men saw more clearly.
"Mr. President, at this time it is suicide, murder, and will lose us every friend at the North. You will wantonly strike a hornet's nest which extends from mountain to ocean, and legions now quiet will swarm out and sting us to death. It is unnecessary; it puts us in the wrong; it is fatal." - Confederate Secretary of State Robert Toombs' reaction to Jefferson Davis' planning to attack Ft. Sumter. Toombs was the only member of the Confederate Cabinet to oppose the attack.
"Let me tell you what is coming. After the sacrifice of countless millions of treasure and hundreds of thousands of lives, you may win Southern independence if God be not against you, but I doubt it. I tell you that, while I believe with you in the doctrine of states rights, the North is determined to preserve this Union. They are not a fiery, impulsive people as you are, for they live in colder climates. But when they begin to move in a given direction, they move with the steady momentum and perseverance of a mighty avalanche; and what I fear is, they will overwhelm the South." Sam Houston, Governor of Texas, shortly after his removal from office for refusing to swear loyalty to the Confederacy.
"You people of the South don't know what you are doing. This country will be drenched in blood, and God only knows how it will end. It is all folly, madness, a crime against civilization! You people speak so lightly of war; you don't know what you're talking about. War is a terrible thing! You mistake, too, the people of the North. They are a peaceable people but an earnest people, and they will fight, too. They are not going to let this country be destroyed without a mighty effort to save it … Besides, where are your men and appliances of war to contend against them? The North can make a steam engine, locomotive, or railway car; hardly a yard of cloth or pair of shoes can you make. You are rushing into war with one of the most powerful, ingeniously mechanical, and determined people on Earth — right at your doors. You are bound to fail. Only in your spirit and determination are you prepared for war. In all else you are totally unprepared, with a bad cause to start with. At first you will make headway, but as your limited resources begin to fail, shut out from the markets of Europe as you will be, your cause will begin to wane. If your people will but stop and think, they must see in the end that you will surely fail." - William T Sherman, President of Louisiana State University