David Lieberman's password is "Zach@ndLeo" which are the names of his kids.
The joke that Billy Russo made about Ann Margret is the same joke made by Pvt. Joker on Stanley Kubrick's Full Metal Jacket (1987), a film that is also about Marines.
The words on the scrabble board in the tent in Kandahar are: die, tits, swarm, armor, fubar, murderous, intent.
Ben Barnes' character, Billy Russo, is reading The Picture of Dorian Gray by Oscar Wilde in the bunk scene. In 2009, Barnes played Dorian Gray in the film Dorian Gray (2009). Both characters parallel the other, as both spends value in his good looks.
The name of the operation Frank Castle is a part of in Kandahar is named "Operation: Cerberus." Jon Bernthal was the character model for Cerberus in the video game, Manhunt (2003).