Massive Attack ’s “Boots on the Ground” doesn’t do much, but it casts a long shadow. Listen even once and you may find its unvarying loops cycling unbidden through your mind, hours afterward—as, say, you stand numbly under the shower or sit blank-eyed at a stoplight: that stumbling trash-can beat, those brooding piano chords, and Tom Waits , sounding more haggard and unhinged than ever, croaking out the song’s titular refrain. Interspersed with that mantra-like phrase, he offers fragmentary visions from the front lines of America’s forever wars as glimpsed through the night-vision goggles of the nation’s jackbooted shock troops: “I kill a brown man I never-ass knew/Choked on spit and then he turned blue.” Waits is one of popular music’s great interpreters of hard-luck characters, but we’ve never heard him play a role like this.

“Boots on the Ground” is a cyclical song about cycles of violence, and part of its genius is the way it collapses multiple theaters of war into one ignominious battle scene, a…