In highschool cluster english in the San Diego City School system
circa 1990 I came across a poem that has stuck with me since, but only
by *meaning*.  I want the full correct text, and author etc., of this
poem.

It is a single-line that relates the experience of a soldier in an
early war, waiting in a ditch with his comrades for the enemy to
charge over a wall.  As the first enemy soldier crests the wall, time
seems to stop for the viewpoint character, and he seems to watch the
bullet from his rifle traverse the distance to the enemy, penetrate
the enemy's heart, killing him, and can watch the tumbling of the
corpse of the first enemy as it falls at the foot of the wall. The
poem's message is carried in the second part: "~The rest fell just
like that.~" (paraphrased)

I suspect an american poet, one or two sentences, writing about WWI (or WWII).