That's really all in what you've been assigned, and how your essay plays out.
Are you looking to contrast two items, in order to explain both in greater detail through their differences? You could potentially be showing similarities between the two to create a link.
Contrast
"Due to the differences in technology, it's availability, and the differing styles of warfare World War II was an incredibly more terrifying war."
Comparison
"Emotions such as greed, jealousy, and fervent nationalistic pride were the causes of both World War I and II."
With a little more information on your essay topic and leeway of thesis, maybe I could help suggest a route. (I won't write it for you though

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