When saying, "Sam joined the marines," does the "m" in marines need to be capitalized?
Yes because it is a proper name for the branch of the military
Proper nouns are always capitalized, so yes the "M" in Marines is uppercase.
The "m" in the word "marines" has to be capitalized. This is because the name is a proper noun from the word Military. You can learn more about proper nouns at www.k12reader.com/proper-nouns/.