The infantry took position with light machine guns. Here the word ‘infantry’ is a/an -

Infantry (noun) 
(সমষ্টিগত singular) [uncountable noun] পদাতিকবাহিনী; পদাতিক।

Collective Noun:
A Collective Noun is the name of a number (or collection) of persons or things taken together and spoken of as one whole.
যে noun দ্বারা সমজাতীয় কিছু ব্যক্তি বা বিষয়ের সমষ্টি বুঝায় তা ই collective noun.
যেমন: Crowd, mob, team, flock, herd, army, infantry, fleet, jury, family, nation etc.