Here's why:
* Iambic: An iamb is a metrical foot consisting of an unstressed syllable followed by a stressed syllable.
* Pentameter: Pentameter refers to a line of five metrical feet.
Therefore, iambic pentameter means a line has five pairs of unstressed and stressed syllables. This is the most common meter used in Shakespearean sonnets and many other English poems.
Let's look at the first line of "Recalling War" as an example:
"The *rain* had *ceased*, the *wind* was *low*, the *sky"
* The rain (unstressed, stressed) - iamb
* had ceased (unstressed, stressed) - iamb
* the wind (unstressed, stressed) - iamb
* was low (unstressed, stressed) - iamb
* the sky (unstressed, stressed) - iamb
As you can see, each line in "Recalling War" follows this pattern of five iambs, making it iambic pentameter.