Interestingly, the intended audience for Wilfred Owen's graphic war poem "Dulce et Decorum Est" is other poets, specifically one poet named Jessie Pope. Owen originally entitled this poem, "To Jessie Pope." Owen directly addresses Pope and her ilk on line 25 where he writes, "My friend, you would not tell with such high zest" the old lie, namely, that it is sweet and proper to die for one's country.
So who was Jessie Pope? She was a British author whose work was published in newspapers and magazines in the early 1900s. Some of her poetry was humorous, such as the verses she published in Punch magazine. However, she was one of the leading composers of a genre known as "jingoistic war poetry." Such poems were used to recruit young men into the military and glamorized war. You can read an example of one of these poems, "The Call," at the link below. After experiencing the horrors of war first-hand, Owen obviously took offense to the flippant recruiting verses that made the military seem like an athletic club. Certainly he had no objection to men signing up to fight for their homeland, but he wanted them to go into it with their eyes wide open.
Although Jessie Pope and others who wrote jingoistic war poems were the primary audience for this poem, the secondary audience was surely young men who were considering enlisting, or current soldiers or veterans who had been tricked into signing up. Those who had not yet joined could think in a more balanced way about their decision, and those who had already succumbed to the bait-and-switch recruitment propaganda could at least feel that someone was expressing their feelings about having been deceived.
A tertiary audience Owen must have had in mind would be the public at large who, by reading his realistic poetry, would have a better idea of the great sacrifice their fighting men were making for their fellow citizens.