Benjamin Britten's War Requiem was composed in 1962 for the rededication of Saint Michael's Cathedral in Coventry, which had been bombed during World War II. It's one of his best and most ambitious choral works, mixing the traditional Latin requiem text with the disturbing war poetry of Wilfred Owen, who was killed in WWI at the age of 25. Text and music combine to offer conflicting worldviews--one serene and optimistic, the other angry and cynical. The brilliantly conceived opening is dark and haunting: a gong sounds ominously, bells chime in the distance, the hesitating low strings gradually gain momentum, then pause, and the choir softly sings, "Requiem, requiem." This sequence repeats again and again with increasing tension until the tenor enters, singing Owen's harsh words, "What passing-bells for these who die as cattle?" Dramatically expressing the horror and destruction of war, the choral symphony ends as it began, with softly chiming bells. Any performance of this work is a substantial undertaking--it's scored for two orchestras (here the Grant Park principals will play the role of the smaller group), piano, organ, a double mixed chorus (the 85-member National Collegiate Chorale of Scotland and the 60-member Grant Park Chorus), a separate boys' choir (30 members of the Chicago Children's Choir), a soprano (Erin Marie Wall), a tenor (Anthony Dean Griffey), and a baritone (Nathan Gunn, who gave an incredible performance in Britten's Billy Budd at the Lyric in 2001). Carlos Kalmar conducts. Free. Friday, August 20, 6:30 PM, and Saturday, August 21, 7:30 PM, Pritzker Pavilion, Millennium Park, 100 N. Michigan; 312-742-4763.