The Great Gatsby, they found the insignia

Could you please tell me if in the following excerpt from The Great Gatsby, chapter 5, "they" in "they found" refers to "the infantry", with a collective meaning, or to someone else:

In the Argonne Forest I took two machine-gun detachments so far forward that there was a half mile gap on either side of us where the infantry couldn’t advance. We stayed there two days and two nights, a hundred and thirty men with sixteen Lewis guns, and when the infantry came up at last they found the insignia of three German divisions among the piles of dead.

Thank you.