The Battle of Shiloh on the banks of the Tennessee River in April of 1862 took place while the war was still relatively young, so to speak, after only a year. During that time, both sides had mobilized armies, redirected their economies towards war production and tested their men under fire at Bull Run and other engagements.
But nothing had prepared them for the bloody slugfest that was Shiloh. During those two days, in that one battle, more Americans were killed and wounded--more than 23,000 total--than in all previous American wars combined. The Battle of Shiloh, therefore, removed any and all doubt about the fact that this would be a long war, and that the level of sacrifice required to achieve anything like victory would be very high. Some even questioned whether or not victory was even possible.
The losses were even more pronounced for Mississippi regiments, given the limits on their numbers, and the rather careless way in which men had been thrown into the breach disillusioned many survivors from staying in the fight, especially since, given that sacrifice, the Union had carried the field at the end of the day anyway.