Why was Northern optimism increased after the victory at Fort Donelson in Across Five Aprils?

The victory at Fort Henry, followed quickly by a second at Fort Donelson, were the first for the Union in the war.  After suffering defeat in the opening battles at Bull Run, Ball's Bluff, and Wilson's Creek, the people "went wild with joy".  Although thoughtful men like Matt Creighton realized that the war, once begun, would be long and grueling, the general public, swept up in the heady euphoria of the North's first decisive victories, were inclined to be overly and unrealistically optimistic, with the expectation that, now that they were winning, the conflict could not help but be soon decided in their favor.

There is no question but that the victories at Fort Henry and Fort Donelson were important.  The two forts were strategically located so that, in taking them, the Northern forces effectively cut off Southern supply lines, dealing the Confederacy a significant setback.  The battles had been won at a great cost in human lives, however, and there were numerous other fronts around the country where confrontations still remained to be resolved.  As Shadrach points out to Jethro later at the schoolhouse, those who were concluding, on the basis of these two victories, that the war would soon be over were sadly naive and mistaken (Chapter 4).