Wilfred Owen establishes assonance in the title and first line and then applies it consistently through the poem. It helps to keep in mind that assonance is oral and not related to spelling. It refers only to vowel sounds, identical or very similar. Repeated or very similar consonants are not part of the device but can enhance the similarity of sound.
Dulce et decorum est pro patria mori.
Bent double, like old beggars under sacks,
Knock-kneed, coughing like hags, we cursed through sludge....
Short E sound: dulce, et, decorum, est, bent, e in beggars.
Long O or related "aw" or "uh": decorum, pro, mori, double, old, a in beggars, under, coughing, cursed, sludge.
Note how several consonant sounds are interspersed in combination with those vowel sounds. This creates rhythm and flow without rhyme: Bent double, B E ... D Uh B L. Old beggars, O L D B E ...Uh....
Also consider this line:
And watch the white eyes...
(The entire section contains 2 answers and 490 words.)