In setting out to write an argumentative essay about Siegfried Sassoon's World War I poetry, you could compare and contrast his work to those of other war poets, such as Wilfred Owen and Rupert Brooke.
One could mention, for instance, that Sassoon shared with Owen a hatred for war, a sentiment notable by its absence from the patriotic poetry of Brooke. It may be useful to point out that such a difference in attitude is largely down to the fact that Brooke's experience of the front was relatively limited, so he had not been exposed to the kind of horrors depicted in the work of his fellow war poets.
As part of your essay, you might like to explore the paradox in Sassoon's poetry of a man fighting bravely in the war but who nonetheless expresses withering contempt and hatred for that war in his poems.
In works such as "Base Details," Sassoon doesn't hold back in expressing his contempt for senior officers. Here, Sassoon presents them as living it up far behind enemy lines while their men are being slaughtered in their droves at the front. The further paradox here is that Sassoon had to follow their orders even though he had such a profound contempt for them.