Why are Marines (US) not "soldiers"?

[robby]

Back to that U.S. Army Style Guide I linked to above. It talks about the term “troops” as well that we discussed upthread.

@U.S. Army Style Guide

troop Lowercase. A troop in its singular form is a group of people, often military or animals. Troops, in the plural, means several such groups. But when the plural appears with a large number, it is understood to mean individuals: There were an estimated 150,000 troops in Iraq. But not: Three troops were injured.