Why did both the north and south think that the civil war would be short?

Both sides fully beleived that their soldiers were better than the others and did not even pause to think they might be evenly matched, or outmatched in some cases. The military leaders also overlooked key military and technological innovations by both sides that prolonged the war. Also, the Northern abolishionists beleived they were doing right in the eyes of God and that he would end the war quickly while some Southerners beleived they were naturally supierior to people in the North and would easily beat them. In short, everyone was wrong.

The last sentence of the gettysberg address