Why did each side in the civil war think that the war would won easily?

Both the Union and the Confederacy believed they would win the Civil War easily due to overconfidence in their respective advantages. The Union, with its larger population, industrial capacity, and extensive railway network, thought its resources would quickly overwhelm the Confederacy. Meanwhile, the Confederacy relied on its knowledge of the land, strong military leadership, and the belief that their fight for independence would rally support and resolve among its soldiers. This underestimation of the other's resolve and capacity led each side to anticipate a swift victory.