“Army” is about her best friend. She described the song on Instagram:
https://instagram.com/p/9dqYRKzfbq/

In it, Ellie talks about this person and how she has always been writing songs about her boyfriends, but not her best friend, who hasn’t left her side for over 10 years.

It’s the third (or fourth, when you look at the premium version) single off of Ellie Goulding’s third album Delirium, it was released on October 29, 2015.