The poem opens with the speaker admiring some lilacs in a dooryard and the "western fallen star." The lilacs remind him of the "one he loved." We later realize that man is Lincoln. The speaker then takes us through a number of funeral processions for fallen soldiers. The entire country is in a state of mourning because of the Civil War and Lincoln's assassination. The speaker lays a lilac upon a coffin as a reminder of life's resilience.
We then end up in a swamp with a hermit bird "warbling a song." Good times. The speaker hangs out for a bit and begins to consider death in a more natural and optimistic sort of way. In turn he creates his own song for death and "the one he loved." Then, we're reminded again of the consequences of the war and all of the dead that litter the land. By the very end we see the lilacs, hermit bird, and western star all hanging out together in the peaceful swamp. Aww.