"The Charge of the Light Brigade" was a poem written by Alfred, Lord Tennyson, a British poet who lived in the 19th century. Its subject was an infamous British cavalry charge at the Battle of Balaclava during the Crimean War (1853-1856).
Just prior to entering battle during Operation Return against overwhelming Dominion and Cardassian forces, Miles O'Brien and Julian Bashir quoted a stanza of this poem. The quote had particular significance because the 600 Starfleet ships were in danger of being outflanked and surrounded by the larger Dominion fleet. (DS9: "Sacrifice of Angels")