The poem begins with the speaker escaping from battle by heading down into a deep, dark tunnel—a tunnel that "titanic wars" had "groined," or cut, into the granite bedrock beneath the battlefield. It's clear right from the start that this poem isn't going to treat war as something glorious or heroic. Note, for instance how the soldier is running away from the fighting. The reference to "titanic" wars also suggests that humanity has had a long, devastating history of violence. This might, in part, be an allusion to that actual ship the Titanic, which had sunk just six years before Owens wrote this poem; as such, the word evokes a sense of destruction and waste on a massive scale.
Note also that the speaker doesn’t say what war he’s fighting in, nor who he’s fighting against. This suggests that the war's sheer brutality and horror overshadow whatever politics started it the first place.
These lines are written in iambic pentameter, as will be the case for much of the poem: each line has, more or less, five poetic feet, each consisting of a da DUM syllable pattern. It's easy to hear this rhythm in the poem’s third line:
Through granites which titanic wars had groined.
These lines also introduce the poem's pseudo rhyme scheme, which will be a combination of slant rhyme and something called pararhyme. This is when words contain the same consonant sounds in the same order (but different vowels) and can be seen with "escaped" and "scooped." The same thing will happen with "groined" and "groaned" in line 4 . Owens was known for his use of pararhyme, especially in this poem, and there will be more and more of these pairs.
The poem's rhymes, such as they are, fall into heroic couplets (rhyming couplets of iambic pentamer)—or, maybe it's more accurate to say failed heroic couplets, where the rhymes fail to fully line up. As their name suggests, heroic couplets are often used to describe heroism and bravery in idealized ways. The poem’s failed rhymes suggest that it is intentionally messing with the form, as if to show the reader what heroic couplets look like after they’ve actually been to a real battle.
The opening two lines of the poem are also enjambed—which gives a sense of the speed of the speaker’s escape from battle. Line 3 is then end-stopped, and from there forward the poem uses a lot of end-stops—which tend to cut off one line from another. There is a sense of separation, of failed reunion, in the structure of the lines (which will eventually be important to the poem thematically).
These lines also exhibit another poetic device that will be important to the poem: alliteration. For example, take the /gr/ sound that appears in "granites" and "groined." The heavy alliteration in the poem gives it a very literary feeling—this poem is not trying to imitate every day, conversational speech. Even as the poem resists literary tradition, roughing up its heroic couplets, it also wants to feel sophisticated.