What happens at Gettyburg? Why did people begin to suspect a conspiracy among the generals in Across Five Aprils?

At Gettysburg, there is a fierce and bloody battle, lasting over the course of three days, between Confederate troops led by Robert E. Lee and Union soldiers under Ulysses S. Grant. The North emerges victorious, but the cost in lives of the battle is obscene. To make things even worse, the victory is incomplete, as

"the beaten army (is) allowed to withdraw and prepare for still more bloodshed, while the victorious army lick(s) its wounds and (makes) no effort to pursue its opportunities." 

This blunder is especially frustrating, as it is not the first time it has happened. Several months earlier, at Antietam, another Union general, George McClellan, had won a telling victory but had allowed the defeated Confederate troops to retreat and regroup, to fight another day. As a consequence, a series of bloody confrontations had followed, and there was no sign that the war was ever going to end.

"With broken young bodies piled high...and thousands of homes rocked in agony over their loss...all over the North people (are) beginning to say, 'What is it - what does it mean? Is there bad blood somewhere? Is there a conspiracy among Northern generals that prevents their following up an opportunity for crushing Lee's army?" (Chapters 8 and 10).