You can discover a lot about a poem by comparing it to one by another author that deals with a similar subject. You could compare features such as theme, form, structure, rhythm, language and figures of speech.
The key thing to do when comparing poems is to note the points where they are similar and the points where they differ. You could make a list noting similarities and differences between the two poems.
Comparison of 'What Were They Like?' by Denise Levertov and 'The Destruction of Sennacherib' by Lord Byron
Both poems look at the effect war has on a specific nation and specifically at the aspect of loss.
The after-effects of war are seen as lasting a long time and affecting more than those who were engaged in fighting.
Both poems were written at a time when war was dominating the news – Levertov's during the Vietnam War and Byron's towards the end of the Napoleonic Wars.
Both poets use similes which compare events to aspects of the natural world.
Levertov uses two blocks of free verse to structure her poem. Byron uses a highly regular poetic form of four line stanzas.
Levertov uses two speakers (the questioner and the responder) to be more openly critical of what has occurred than Byron’s speaker who is more of an observer and recorder of events.
Byron's tone is more open to interpretation than Levertov's even though both raise questions about the destructiveness of war. In Byron's poem the sympathy of the speaker could lie with either side, but Levertov has sympathy for the Vietnamese people.
Byron uses more figurative language throughout the poem, but Levertov restricts this to the second half to show a contrast with the straightforward and plain questions in the first half. There appears to be sadness for the loss of the culture.
The reader has the sense that Byron tells a complete story but what happens in Levertov's poem is more open-ended.