The poem reads like a series of snapshots from a scene of violence, seen from a distance. People stand on an iron bridge watching. Around them come flat rat-a-tat sounds, whistling objects, cratered paths and shattered glass. The poem never names a war, but the imagery strongly suggests gunfire and shelling, and the title, "Propaganda 39," hints that we may be getting a managed or filtered version of events.
The middle section pulls out five words (spectators, flat, whistles, paths, glass) and scatters them across the page, as if the scene has broken into pieces. Then the opening repeats with changes. The spectators are now "intoxicated," the rat-a-tats become "flat ums and ahs," and the whistles turn "melodious." Everything has been softened, almost made pleasant.
That shift is what unsettles. The speaker seems detached, even numb, and the mood is cold and eerie. Horror becomes something people get used to, watch, and perhaps enjoy.