Clark's enthusiasm for the war waned considerably after the Battle of Fredericksburg during which the Union Army suffered severe casualties in repeated charges against a heavily fortified stone wall. In a January 1863 letter to a friend, Clark wrote that, although he still felt "the principles for which we fight are right and honorable," he was "disheartened and dissatisfied" with the government and the army. By April 1863, the numbers of the 21st Massachusetts had been so thinned by what Clark called the "cruel fate of war," that the regiment had virtually ceased to exist and Clark's command was only nominal. He therefore resigned his commission and returned to Massachusetts.