Based on the non-fiction book Imperial Life In The Emerald City by Rajiv Chandrasekaran, the film is set in and around the Green Zone, the base of operations for US troops in Iraq. Brian Helgeland has adapted the book for the screen; the cast also includes Matt Damon, Greg Kinnear, Amy Ryan and Khalid Abdalla.
Isaacs will play military officer Lt Briggs in the film which is currently shooting in Morocco. It will move to the UK later this year.
Tim Bevan, Eric Fellner, Greengrass and Lloyd Levin, who all produced Greengrass' 2006 hit United 93, are producing.
Isaacs, who previously worked with Greengrass on BBC TV film The Fix in 1997, recently completed Good opposite Viggo Mortensen for director Vicente Amorim. He will start shooting the third season of Showtime series Brotherhood this summer.