Alliteration is a key feature of "Attack." It is first used in the very first line, with the shared /d/ of "dawn" and "dun." Though these words are far apart, the similar consonant /n/ sound makes the alliteration helps more prominent, so that the words actually half rhyme. The alliteration here works as a kind of contrast: "dawn" is often, in poems at least, a visually beautiful time of day. But the poem's first color is "dun"—which is a kind of brown-grey. This hints at the way that war has had (and continues to have) an adverse effect on the natural environment.
Alliteration is then an obvious feature of lines 3-4. These /s/ sounds—also known as sibilance—evoke the smoky battlefield. They dominate the line, preventing the chance for other sounds to appear (mimicking the concealing effect of the smoke):
Smouldering through spouts of drifting smoke that shroud
The menacing scarred slope;
The alliteration of /b/ sounds across lines 6 and 7 ("barrage," "bowed," and "bombs") has a loudness to it that suggests the noisiness of the battlefield, with bombs falling all around. Later, in line 9, there is a kind of double alliteration:
Lines of grey, muttering faces, masked with fear
These repeated sounds create a sense of numerousness to match the anonymous mass of soldiers going "over the top."
Finally, line 11 uses alliterating /t/ sounds to suggest the sound of a ticking clock or watch: "time ticks." This is intentionally very close to the usual way of depicting a clock's sound: tick tock.