I am not sure what you mean by “fog of war” because this phrase does not appear in this poem by Wilfred Owen, but it certainly is a phrase that has been applied to war and may be evident in the purpose of this poem.

The term “fog of war” usually refers to unplanned things that happen as a result of war or fighting. For example, if someone is killed by mistake or by “friendly fire”, this is often blamed on the “fog of war”. The term means that there is so much chaos going on, that clarity is compromised, and things often appear “foggy”. One’s perspective can be “dimmed”, or one’s judgment.

This poem is one of Owen’s anti-war poems, for which he was noted. He wrote many poems as a result of the horror he experienced during World War I. In this one, it is a sonnet, dedicated as an “anthem” for soldiers. The imagery, in typical Owen fashion, is stark – the soldiers are “dying like cattle” is a harsh simile; “the monstrous anger of the guns” is a strong example of personification. The voices of those that mourn for them are “shrill demented choirs of wailing shells” – a vivid metaphor.