Answer: The underlined phrase ("against an enemy") is modifying the words "is used".
Explanation: "Against an enemy" is a prepositional phrase. A prepositional phrase is a group of words that includes a preposition, an object and, in some cases, a modifier. Moreover, in general, a prepositional phrase modifies a noun or a verb. "Against an enemy" is made up of the preposition "against" and the object "an enemy". Furthermore, it is modifying the verb phrase "is used"; therefore, in this case, the prepositional phrase is behaving adverbially.
