The meter is iambic pentameter, usually regarded the most frequently used metrical form in English poetry. It consists, of course, of five poetic "feet" per line, each foot being an iamb, an unstressed syllable followed by a stressed one.
As in most poetry, Owen varies the exact pattern from time to time. This is generally considered necessary because if every line follows the exact same pattern, the verse can become repetitive and simplistic, like "doggerel," or substandard poetry. For example,
Men marched asleep. Many had lost their boots.
In this line the word "many" interrupts the normal pattern because it's accent is on the first syllable rather than the second. Another change is in the line
Of gas-shells dropping softly behind.
Here the line is truncated, containing only nine syllables instead of ten. Poets make these interruptions of the standard meter not merely for variety, but to emphasize something of special importance. Here because "gas" is mentioned for the first time, Owen...
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