Why did the war last for four years proving this assumption false in 1861?

The assumption that the Civil War would be short and decisive in 1861 was proven false due to several factors, including the underestimation of the resolve and resources of both the Union and the Confederacy. Initial battles revealed that military tactics, logistics, and the scale of the conflict were more complex than anticipated. Additionally, political divisions, the emergence of strong military leaders, and the deep-rooted social and economic differences between the North and South prolonged the war. As a result, the conflict evolved into a grueling struggle that lasted four years.