"Dulce et Decorum Est” is written in iambic pentameter. It is not perfectly regular and there are substitutions of other kinds of metrical feet, but the rhythm is consistent enough to be called iambic, and there are five feet per line.
An “iamb” is a kind of metrical “foot” consisting of one unstressed syllable followed by one stressed syllable. An example of an iamb would be the word “delay.” We emphasize the second syllable in this word and not the first syllable, pronouncing it de-LAY and not DE-lay. The word “pentameter” means that there are five of these metrical feet per line, because the prefix penta- means five. Thus, most lines in the poem have ten syllables total.
Let’s look at the first four lines of the poem. I usually begin the process of figuring out a poem’s meter by starting with the words that have more than one syllable. Then, I fill in from there. I will put each stressed syllable below in bold, and I will separate the feet from one another with a “|” mark.
Bent dou | ble like | old beg | gars un | der sacks
Knock kneed | cough ing | like hags | wecursed | through sludge
Till on | the haun | ting flares | we turned | our backs
And towards | our dis | tant rest | be gan | to trudge
We can see that lines 1, 3, and 4 are regular and do not deviate from the iambic metrical pattern; however, line 2 is different. The first two syllables are stressed, and this metrical foot is called a spondee, which consists of two stressed syllables in a row. This spondee is substituted for the first iamb, and the second foot in the line is called a trochee, which consists of one stressed syllable followed by one unstressed. These substitutions, or metrical variations, draw our attention to these words, via their sound, emphasizing how very old and weak these youthful soldiers seem as a result of the way war has ravaged their bodies.