The line "one morning two butterflies play in front of our trench"is an example of

Personification

Explanation:

Personification is the act of conferring human characteristics on inanimate objects or on the abstract, such as emotions and animals, for example.  The personification is much explored in literature, especially in stories of fantasies and fables. In this way, the narrative becomes more expressive, dramatic, poetic and creative, qualities that are essential for the construction of an interesting literary text.

In the case of the line "One morning two butterflies play in front of our trench", the personification was used in the attribution of a human characteristic (the ability to play) to the butterflies.

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