1. [n]  a minor show that is part of a larger one (as at the circus)
2. [n]  a subordinate incident of little importance relative to the main event; "instruction is not an educational sideshow"

Sideshow
In Sideshow, journalist Shawcross presents the first full-scale investigation of the secret and illegal war the United States fought with Cambodia from 1969 to 1973, paving the way for the Khmer Rouge massacres of the mid-70s.

more details ...