Even though our speaker never actually mentions it, "Beat! Beat! Drums!" is a poem about the American Civil War. Sure, we don't see images of battlefields and flying bullets, but what do get are the effects of war away from the battlefield: the disruptions it causes, the way it leaves no one unaffected. In this poem, the music of the drums and bugles carries the word of war to the American people. This is a war that can't be explained, can't be stopped, and can't be controlled: but at the very least, our speaker says, we can pay it some attention.
Our speaker isn't concerned with either promoting or criticizing the war: he just wants to make sure everyone understands what a big deal it is.
By describing the way that the music leaves no one in peace or happiness, our speaker is clearly criticizing the war as a negative force. This guy is clearly anti-war.