Meryl Streep is arguably America’s greatest living musical-theater actress. Anyone who saw the two-time Oscar winner shamelessly mug and prance through the mindless movie musical Mamma Mia! earlier this year might call me certifiable, but those who caught her Mother Courage in Central Park two years ago would probably agree. John Walter’s new documentary, Theater of War, has many virtues, but, most important, it preserves bits of that performance for posterity. Walter uses the Public’s production not just as a backstage docudrama, but rather as a jumping-off point for a meditation on Brecht, performance, Marxism, and war. Walter’s clear inspiration for Theater of War, subtitled Five Acts About Bertolt Brecht, is Errol Morris’s Fog of War: Eleven Lessons From the Life of Robert S. McNamara, which Walter emulates not just in name but also in structure and Robert Miller’s ominous, minimalist, Philip Glass–like score. The problem is that while Morris’s film revolved around one man, Walter’s doc can’t decide whether it wants to be about Brecht, Streep, or Mother Courage herself. What it says about all these subjects (especially in a sequence devoted to Brecht’s second wife, Helene Weigel, the first actress to play Mother Courage) is usually interesting and elegantly presented, but what ultimately stands out is Streep’s performance of Brecht’s “Song of the Great Capitulation.” Few actors can act while they sing as effortlessly as Streep, and these minutes (which Walter shows as one long take) are almost powerful enough to carry an entire feature doc by themselves—and wipe the horrors of Mamma Mia! from memory.