The title gives the poem away: the speaker is an "armchair activist," someone who cares about a war but acts on that concern from a safe distance. The poem is only a few lines long, and it quietly mocks its own speaker.
Notice how small the "activism" is. The speaker "practices" objection, as if it were a hobby or an instrument to be rehearsed rather than something done. They "give ear / to an ear / to the war," which suggests listening to news or someone else's account, not hearing the war itself. Even the sound of it, "a din after dinner," arrives once the meal is finished and the speaker is comfortable. Then they tidy up, "put things in order," a domestic gesture that sits oddly beside the chaos of war.
The final lines, "content / to practice," land with a sting. The speaker seems satisfied with merely having opinions. The mood is dry, uneasy, and self-aware, with a hint of guilt beneath the calm.