Here is a link to the poem.
A bit about the form of the poem, because it is important to its meaning. This poem rhymes, often with loose rhyme, but it does. The stanzas are six lines each, and the rhyme scheme is a-b-c-c-b-a.
Think of the rhyme scheme as a progression; it goes up, reaches the top, then goes back the way it came. An arc, perhaps. Or--a parabola.
Bullets and shells travel in an arc. Because mass loses velocity over time and distance, all projectiles are launched upwards (even slightly, in the case of a rifle), because they will fall as they travel out towards their target.
So do balls. Children's balls. And so the poem begins: "Under the parabola of a ball..."
It's a simple transition for the poem's speaker to go from children's games to the game of killing a man during war. It makes the reader pause to think how difficult it should be.
For me, the most memorable line in the poem is this:
"How easy it is to make a ghost."
In the final stanza, with its comparison of a human life to the image of the shadow of a mosquito, I find echoes of Shakespeare's Macbeth ("life is but a walking shadow...").
This is a beautifully crafted poem about a sobering subject.
To add a layer of pathos to all this, Keith Douglas was himself killed in battle--at Normandy, following D-Day.