This poem has 14 lines all right, and a complex rhyming scheme, but it is not in iambic pentameter, and it does not seem to divide up naturally into an octet and a sestet, nor does it have a 'turn' either at line 9 (like a Petrachan or Italian sonnet) or just before a final couplet (as a Shakespearean or English sonnet would). Instead it has two apparently 'mirrored' stanzas of 7 lines each, and in some ways defys the sonnet form--enacting its title and finding no solution to the problem posed, but rather an increased sense of frustration or futility. This sense that it is in some ways a 'static' sonnet is interesting, I think, because I suspect it is a deliberate choice by Owen.
The poem briefly recounts an incident in the war--a soldier is found in the morning to have died overnight in his bunk in the trenches. Moving him (quite practically) out of the sleeping area provokes a thoughtful meditation on the pointlessness of war. Although we are not explicitly told what has killed the soldier, the implication that he has been unharmed 'until this morning, and this snow' might lead us to think that he has died of exposure and cold. One of Owen's other well-known poems is called 'Exposure', written in 1917 (the year before 'Futility') and this vividly depicts the cold endured by the troops, something which might make you lean towards this reading. In 'Exposure', the very thought of the sun seems almost like a touchstone against the cold. Owen suggests that the reason that they endure the misery of cold and ice in the trenches is, in effect, to keep the home fires-and the sun-- burning:
We turn back to our dying.
Since we believe not otherwise can kind fires burn;
Nor ever suns smile true on child, or field, or fruit.
This thought seems to be echoed in 'Futility', except here the sun which implicitly smiles on children back at home is asked to shine on the man who has died.
The poem starts with an imperative--one that could be seen as a simple command--but the use of 'gently' immediately turns the simple act of moving the body into a gesture of tenderness. the sun is immediately personified with the ideas of 'its touch...whispering', and this personification is developed into the idea of 'the kind old sun' seeming to echo both the 'kind fires' and the 'suns smile true' of 'Exposure'. It is implied that the soldier used to be a farmer--someone who woke with the dawn, knowing that there was work to do, with 'fields half-sown'--an image of growth and new life that is also developed in the poem. The soldier, we are told, stayed in the habit of waking with the sun 'even in France', and the plaintive 'always it woke him' seems to emphasise the central fact that he will never wake again. Throughout the poem, Owen plays with the word: 'awoke...woke...wakes...woke' (in a rhetorical figure called polyptoton), with a move into the present tense at the start of the second stanza that seems to promise progress and resolution--until it is disappointed (interesting that the sound is also caught up with the internal rhyme with 'break' in the final line of the poem).
The half-rhymes, or pararhymes 'son/sown', 'once/France' are very typical of Owen and create a subtle effect whereby the tone of the poem seems conversational, the long vowels contributing to the gentle effect of the first stanza. The second stanza, by contrast, seems harsher, with the move into the present tense accentuated by the imperative 'think' and the harsh 'k' alliteration (think..wakes...woke...clays...cold') alternating with sibilant 's' sounds and shorter vowels to speed the poem's pace.
This increasing pace leads up to the series of questions that form the climax of the poem. The image of the sun's power to bring life to 'the clays of a cold star' (to draw life from the earth) suggests that it is being lazy or uncaring in not waking up the man (I'm tempted to think here that he is half-remembering John Donne's poem 'The Sun Rising' which starts: 'Busy old fool, unruly Sun, / Why dost thou thus,/
Through windows, and through curtains, call on us ?') The bitterness of the question 'Are limbs...sides...still warm too hard to stir?' implies the answer 'no, they are not' and yet we know rationally that the sun cannot awaken a dead man, increasing the sense of frustration felt.
The image of 'clay' picks up on the traditional idea (in the book of Genesis) that mankind was made by God from the earth (the word 'Adam' comes from the same root as the word for 'earth' in Hebrew, and is also connected to the word for 'red', meaning that Adam was seen to have been created out of red earth, or clay). Of course, clay also needs heat if it is to become a creation of pottery; 'cold clay' is opposed to life and creativity in this sense. The idea that 'the clay grew tall' refers to the idea that mankind has gained ascendancy over the world--'taller' in effect than the rest of creation. Here. the question is clearly bitterly rhetorical--the implicit answer is 'no, mankind did not develop just for this--for war for death and destruction--and yet of course death is ultimately the end of all human beings, no matter how beloved or accomplished.
Owen's final bitter question changes his attitude to the sun completely--now the sun really is 'foolish' as Donne suggested, it is 'fatuous' and idiotic, working pointlessly 'to break earth's sleep'. By the end of the poem, Owen finds no resolution, but only a deeper frustration.
To-night, this frost will fasten on this mud and us,
Shrivelling many hands, puckering foreheads crisp.
The burying-party, picks and shovels in shaking grasp,
Pause over half-known faces. All their eyes are ice,
But nothing happens.
Kenneth Simcox has a thoughtful reading of the poem at the Wilfred Owen Association which is well worth a look.