Ewing's wartime service was characterized by several incidents which would have a unique impact on history. In 1861, his political connections helped save the reputation of his brother-in-law, William T. Sherman, who went on to become one of the north's most successful generals. Ewing himself went on to become Sherman's most trusted subordinate. His campaigning eventually led to the near-banishment of Lorenzo Thomas, a high-ranking regular army officer who had intrigued against Sherman. He was present at the Battle of Antietam, where his brigade saved the flank of the Union Army late in the day. During the Vicksburg campaign, Ewing accidentally came across personal correspondence from Confederate President Jefferson F. Davis to former President Franklin Pierce which eventually ruined the reputation of the latter. Ewing was also present in Kentucky during Major General Stephen G. Burbridge's "reign of terror", where he worked to oppose Burbridge's harsh policies against civilians, but was hampered by debilitating rheumatism. He ended the war with an independent command, a sign he held the confidence of his superiors, acting in concert with Sherman to trap Confederate Gen. Joseph E. Johnston in North Carolina.