In just six short lines, the poem turns war into a kind of game. The speaker says that "with nothing else" to give, we hand the general a field. It sounds almost like a gift, or a bleak shrug, as if a patch of empty ground is all a commander needs to begin his work. The soldiers are then "to lie" in it, a phrase that suggests both resting and being dead, and the final image compares them to "played cards."
That comparison is the heart of the poem. Cards are dealt, spent, and tossed down without a thought for any single one. The soldiers become pieces in the general's hand, and the "we" in the poem seems quietly complicit, since we are the ones who supply the field.
The tone is flat, controlled, and cold. The speaker doesn't shout or mourn openly, and that restraint makes the poem feel more unsettling. The mood is one of quiet dread and bitter irony.