So, if your thesis (DEVICE-BASED APPROACH) is:
In his poem "Dulce et Decorum Est", Wilfred Owen portrays the atrocities of war through his use of connotative diction, structure, and auditory imagery.
Then your topic sentences will sound something like this:
TS for body 1:
Through the use of connotative diction, Owen conveys the idea that war is truly terrible.
TS for body 2:
Owen's use of structure in "Dulce et Decorum Est" likewise helps to display the horrors of World War One to readers.
TS for body 3:
Auditory imagery throughout the poem also serves to highlight the atrocities of war.
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THEME-BASED APPROACH:
Sample Thesis:
In "Dulce et Decorum Est", Wilfred Owen presents the atrocities that soldiers experienced in World War One and also provides a harsh criticism the war throughout the poem.
TS for body 1:
Owen highlights the horrible realities that WW1 soldiers experienced throughout "Dulce et Decorum est" with his usage of connotative diction, visual imagery, and syndeton.
TS for body 2:
Owen uses "Dulce et Decorum Est" not only to point out the war's atrocities; he also exposes war's realities and criticizes its purpose and typical representations through his tone, auditory imagery and sibilance, as well as strong juxtaposition.