Answer:
Metaphor
Step-by-step explanation:
Simile? No, the sentence isn't using like or as
Metaphor? Yes, the cavalry is, "used to refer to a source of help or rescue in an emergency, especially as a last resort." So it is comparing cavalry to help.
Rhetorical question? No, its not a question
Counterargument? No, there is not an opinion within this phrase.