"Sweet Polly Oliver" is an English broadside ballad (Roud #367), traceable from 1840 or earlier. It is also known as "Pretty Polly Oliver" and has several variant sets of lyrics, set to a single anonymous melody.
It is one of the best known of a number of folk songs describing women disguising themselves as men to join the army to be with their lovers.
Thomas Root wrote a symphonic band arrangement and Benjamin Britten wrote an arrangement for voice and piano.