1862, Sladen joined the Union Army for the American Civil War, enlisting as a private in Company A, 33rd Massachusetts Infantry Regiment. He took part in several of the regiment's engagements, including the Battle of Chancellorsville and Battle of Gettysburg. Sladen participated in the May 13-15, 1864 Battle of Resaca
<unk> <unk> While serving as a clerk at the regimental headquarters, Sladen left his post to voluntarily join the battle.<unk> While serving as a clerk at the regimental headquarters, Sladen left his post to voluntarily join the battle. ąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąą 1895, he received the Medal of Honor in recognition of his Civil War heroism.