As others have pointed out, trenches were widely used during much of the Civil War, usually in sieges. Trenches played a major role in the Seven Days, Iuka-Corinth campaign (Sept '62), Vicksburg, Chattanooga, Spotsylvania, Cold Harbor, the long Petersburg campaign, Atlanta, and Hood's Tennessee campaign. read more
The American Civil War did end up in trench warfare. The last major battle of Petersburg was a battle of trench warfare. Grant learned the lesson of the futility of charges against fortified works at Cold Harbor, the last Union disaster. read more
Trenches remained merely a part of siegecraft until the increasing firepower of small arms and cannon compelled both sides to make use of trenches in the American Civil War (1861–65). The trench lines of the Petersburg–Richmond theatre of operations in the final months of that war were the foremost example of trench warfare in the 19th century. read more