The first poetic device Hardy uses is rhyme. The scheme is ABAB. The shortness of the lines and the repetitive rhyme scheme gives the poem an easy cadence, making it easy to recite and remember, and gives it a fun type of lightness. The lightness is significant because the subject matter is both dark and light. The infantryman gives a monologue in which he suggests that if he had met his enemy at a bar, they would have been friends.
The famous line of the poem is an understatement: "quaint and curious war is!" The use of understatement (opposite of hyperbole) goes along with the lightness of the poem. And, of course, it undercuts or makes light of the fact that war is deadly. One could say that Hardy is being sarcastic or ironic here because war is certainly not "quaint." Quaint can mean bizarre or interesting, but often in an old-fashioned way. War is bizarre but in a violent way. So, to downplay war as quaint and curious is a kind of "verbal irony." This is when the speaker intends a meaning different from the one he expresses. Hardy says war is quaint and curious but within the context of the poem, he is making the point that it is devastating. He makes this point by using a contrast between the two situations: the two men at a bar vs. the two men on the battlefield.