In just a few lines, the poem makes a quiet, bitter accusation. The first sentence says that leaving red out of a picture of war, with no blood and no visible violence, is a "white lie." The pun works well: white is the colour of a harmless fib, but also the colour of something bleached and erased. A war told without blood is a war told dishonestly.
The second stanza turns to the people who pay for it. "Thin as corn" suggests stalks standing in a field, and also bodies worn down by hunger. The title, "Harvester," adds a darker thought: if something is reaping here, it is reaping the poor. The speaker doesn't shout, and the tone stays flat and controlled, which makes the anger feel colder and more serious. The mood is stark and sorrowful, and it leaves you uneasy about what gets left out when war is described politely.