How do the comparisons in lines 1, 14, 20, and 23-24 contribute to the effectiveness of "Dulce et Decorum Est"?

The comparisons you refer to are notable for the way that
they deliberately attempt to present war in an incredibly brutal and realistic way.
Given the title, and its presentation of soldiers, we automatically summon up an image
of smart soldiers in their uniform marching together and bravely fighting. The image in
line 1 therefore deliberately contradicts our expectations by presents the soldiers as
being "like old beggars under sacks." There are no smart, young, brave soldiers here,
but only men who have been prematurely aged by their
experiences.

The image in line 14, when the speaker sees
his fellow soldier "drowning" through the gas mask likewise challenges our expectations
of soldiers. We expect soldiers to meet their deaths in battle fighting against the
enemy, but here we see the speaker's friend dying ignomoniously away from the from the
front line by a gas attack, and "drowning" rather than dying in hand to hand
combat.

In line 20, we are given an incredibly grim image
of the corpse as "the hanging face" is described as being "like a devil's sick of sin."
No noble, glorious death for this soldier, only a death that horrendously disfigures his
body with his sufferings.

Lastly lines 23-24 emphasise the
pain and agony that this dead soldier endured by focusing on how terrible his death
was:

Obscene
as cancer, bitter as the cud

Of vile, incurable sores on
innocent tongues--

All of
these images are therefore alike in the way that they profoundly challenge our ideas and
expectations of soldiers and battle, presenting it as a terrible, dehumanising and
demeaning experience involving tremendous suffering and no glory whatsoever. This of
course supports the main message of this excellent poem.