Three short lines, and a lot of quiet weight in them. The poem is about remembering the dead, and the poppy, the flower so often worn for remembrance, is at the center of it. "Poppies buried with" breaks off at the end of the line, so the flowers feel placed in the ground alongside the dead, as if the sentence itself were being laid to rest. The next line shifts to "we," which suggests the speaker is part of a community of mourners, not a lone observer.
The last line is the most open to interpretation. A white flag usually means surrender or a plea for peace, so "No white flags remain" could mean the fighting is over, or that no one gave in, or that there is no peace left to signal for. The poem doesn't settle it, and that uncertainty is part of its pull.
The mood is solemn, restrained and a little stark. The speaker doesn't explain their grief, and the short, spare form lets the silence around the words do much of the work.