Final answer:
The increase in desertions was the main reason for the decrease in the number of soldiers during the Civil War, compounded by internal dissent and the shift to total war requiring more manpower.
Step-by-step explanation:
The main reason for the decrease in the number of soldiers during the Civil War was desertion, which began to increase as the war dragged on. Initially, both the Confederate and Union armies had high levels of voluntarism, but as the reality of a long and grueling war set in, the energy and zeal of soldiers waned. Desertions were a significant problem for the Confederate army, fueled by a variety of factors, including disillusionment with the war, class divisions, and disagreements over state versus central governmental powers.
Conscription laws were passed in an attempt to bolster the ranks, but they were met with resistance, and states often hesitated to comply with central government requests. Not only did the South face internal challenges and dissent within the Confederacy, but the war also evolved into a form of total war, with both sides seeking to completely demoralize the enemy and disrupt their ability to fight, leading to an even greater need for manpower. This combination of factors made it increasingly difficult to maintain high soldier numbers as the war continued.