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There are several items that identify Gillian Clarke's, "Lament," as an anti-war poem, conveyed in the tone.
The images Clarke presents show the grisly and tragic fallout from war, not only upon the land and the animals, but upon man as well. These observations come from photographs provided by the media. As this is a lament, the imagery denotes tragedy and loss, and this is the tone of the poem.
The first example speaks to death: "For the soldier in his uniform of fire." Clarke, in this line, mourns the death of a solider who was set on fire when his tank exploded.
The second example mourns the harm to the earth: "The veil of iridescence on the sand" describes the oil washing up on the shores of Kuwait from the bombing of oil wells. This not only poisons the waters, but the animals that depend on it to survive.
The final example grieves for the animals harmed by the fighting: the media provided pictures of cormorants covered with oil, described in the line "in his funeral silk," which would speak to the death of animals smothered by the "spilling" oil in the ocean.
Because this is called "Lament," which is a form of mourning or grieving, and "for" starts each line that lists another "victim," we can see how Clarke speaks out against the harm and devastation that accompany war.
The tone, therefore, is sad and grieving. The poet's horror is apparent in the images she describes.