Harper's Ferry, 13-15 September 1862
Vermont units present/engaged: 9th Infantry
The entire regiment was briefly captured.
See Captain E. H. Ripley's rendition of the battle in Volume 2, Chapter XXII of George Benedict's "Vermont in the Civil War," starting on page 191.
Battle of Harper's Ferry (Wikipedia)
Harper's Ferry (CWSAC Battle Summaries)
Harper's Ferry (American Battlefield Trust)