That is another story. But the application is that to maintain and army you must maintain moral. And one of the main way to maintained moral is for the soldier to believe that his sacrifice is not being wasted in vained. Hence the pointless bayonet charge would quickly break a soldiers moral if that were the only tactic used.
Doyle, I would submit that most Union soldiers never believed their sacrifices were in vain, as evidenced by the fact that the Army endured horrific early losses to ultimately triumph. The morale issue was a problem of leadership, not cause. While many Confederates stuck it out to the bitter end, many others deserted the field in droves when it became clear that the Noble Cause was inexorably going to flat-line. I think you have to give an Army a lot of credit that constantly got drubbed but stayed the course. Like old Nathanael Greene said in an earlier day: "We fight, get beat, rise and fight again."