Even though we're not on a battlefield in this poem, we can hear the sounds of war loud and clear. For the most part, these sounds are carried by the drums and bugles, reminding us that we can't just expect to live our normal lives when the war is happening around us. In their shrillness and terrible power, these instruments are the closest the American people would like to get to war.
Line 2: The simile "like a force of armed men" makes a clear connection between the sound of the instruments and the violence of war. Already in line 2, we're pretty sure that the instruments are supposed to represent the war itself (or at least carry its message). There's also an implied metaphor in that word "burst," suggesting that the music is something that can explode through windows and doors. Like a bullet or a cannon ball.
Line 8: Yep, this is line 8, but also like 1 and line 15. It's what we call a refrain, repeating at the start of each of the three stanzas. This repetition totally emphasizes the repetitive, insistent sound of the drums and bugles.
Line 7: Our speaker uses a lot of onomatopoeia to deliver the sounds of war to us. Here, the drums "whirr" (and "pound"): it sounds like they're gearing up to fight.
Line 14: The onomatopoeias are back: now the drums "rattle." This is a little more forceful than the whirring before, and we can feel the war getting closer.
Line 21: Finally, the drums "thump" (oh, and they're "terrible"); the war is approaching (the sounds get louder and louder) and our speaker seems to telling us that we shouldn't – and can't – escape it.