Final answer:
The true beginning of the Civil War was marked by the Confederate forces' attack on Fort Sumter on April 12, 1861, an event that highlighted the severe underestimations both the Union and the Confederacy had about each other and the war's duration.
Step-by-step explanation:
The assertion that the assault upon a military outpost marked the beginning of the Civil War is indeed true. The defining incident was the bombardment of Fort Sumter by Confederate forces on April 12, 1861. This attack initiated a conflict wherein both sides underestimated the other: the Confederacy the Union's resolve, and the Union the Confederacy's anger. As Confederate forces laid siege to Fort Sumter, it became clear that the war would be neither brief nor devoid of suffering. The first major battle, First Manassas, further solidified the realization that the conflict would be long and brutal. The North's superior resources of men and supplies posed a significant advantage over the South, which would have needed a swift victory to secure any chance of success.