Near the end of the Barry Manilow musical Harmony, the surviving Comedian Harmonist Ari Leschnikoff, “a Bulgarian singing waiter” who survived the Holocaust to return to his home country, brags to a rabbi: “We saved them, Rabbi! Every Jewish person in Bulgaria! We wouldn’t let them have them! Not one!” This startling declaration, which demanded elaboration, is the foundation of The Brief Life & Mysterious Death of Boris III, King of Bulgaria. The story of Boris III might have remained a historical footnote but for Sasha Wilson, the cowriter of the piece (with Joseph Cullen), whose grandparents escaped Bulgaria during World War II. It turns out that the history of Bulgaria in the 20th century is far more complicated than the Harmony passage suggests.