The poem's meter is primarily iambic pentameter, which is the style Shakespeare used for his plays. In iambic pentameter, each line has 10 syllables. The syllables alternate in an unstressed/stressed pattern, beginning with the unstressed syllable. For instance, this line
Till on the haunting flares we turned our backs.
would look like this if we wrote it to show the meter:
Till on/ the haun/ ting flares/ we turned/ our backs.
However, sometimes Owen deviates from this metrical pattern, as in these lines, which have 11 syllables each:
“GAS! Gas! Quick, boys!-- An ecstasy of fumbling,”
“As under a green sea, I saw him drowning.”
Perhaps Owen uses this break in the pattern to emphasize that war is not "sweet and fitting."