Q. I am writing a poem with the phrase “marine helicopters”: “In the silvery drizzle, / a pair of Marine helicopters / flying low on maneuvers . . .” These are indeed helicopters of the United States Marine Corps. Would “marine” or “Marine” be correct in this circumstance?
A. In documents published by or for the US Marines, it’s “Marine,” with a capital M, whether the term is singular or plural and regardless of the part of speech. But outside the Marine Corps (and unless the focus of the text is the US military), the word “marine,” like “navy” or “naval,” can usually be lowercased when used by itself as a noun (“a marine,” “the marines”) or adjective (“a marine helicopter”).
In poetry, you have options. If you want to nudge your readers toward interpreting that line as referring to the US Marines, then a capital M is probably best. But if you’d like to retain a note of ambiguity, leaving the term more open to evoking the colors of the sea and the like, you could go with a lowercase m. That’s up to you.
See also CMOS 8.113, which treats capitalization conventions for armies, battalions, and the like, including the US Marines. For “US” (normally spelled without periods in Chicago style), see 10.4 and 10.37.