The growing prominence of private military firms was brought sharply into focus during the 2003 invasion of Iraq when the US forces were joined by an equally large army of commercial contractors. This privatisation of war brings a wide variety of complicated issues, the most prominent of which is to do with who or what actually holds these commercial players to account when things go wrong.

It was against this backdrop that EA Montreal released co-operative third-person shooter Army of Two (2008), in which players became mercenary fighters in a variety of war-torn locations, including Iraq during the invasion. Not surprisingly, the game struggled to balance such moral issues as those above, and was duly admonished by the critics. However, the series is now back with a sequel, titled Army of Two: The 40th Day, which will hopefully bring a more even-handed take on the world of private military personnel.

This game's main protagonists are two guns-for-hire named Tyson Rios and Elliot Salem, who have established their own commercial military firm called TransWorld Operations. Aided by handler Alice Murray, the two men happily dive into situations where others would fear to tread in pursuit of a big pay day if they can dodge the bullets.

Set a few years after events in the first game, the sequel sees Rios and Salem operating out of Shanghai, China. After a rival military contractor attacks the sprawling metropolis, the two mercenaries are hurled into a survival situation as buildings tumble down and the city descends into chaos. EA Montreal's creative director Alex Hutchison said that story will force players to deal with "anything and everything" as the world collapses around them.

Hutchison explained that Army of Two: The 40th Day's "defining" feature is that it caters for "every flavour" of co-operative play, including split-screen or single player with an AI-controlled teammate over the local network, or online play over the PlayStation Network or Xbox Live. Whichever mode is chosen however, playing as a disciplined team will be an essential facet of surviving the destruction of Shanghai.

Alongside a range of heavy duty weapons, Rios and Salem can also use the environment and psychological tricks to achieve a favourable outcome. For example, one of the two men can feign surrender to buy some time while the other gets into a flanking position to take out the enemies. Establishing the richness, variety and quality of the co-op experience has been the development team's biggest challenge.

"We support all the co-operative modes and it's been a massive headache to do so, but we wanted everyone who wants to play the game with a buddy to find some way to do it," according to Hutchison. "We were doing some research early on in the development by going to Shanghai and Korea to take pictures. We ran across a bunch of guys at an army base who were big fans of the first game. It was the only game that they had and they were playing on a tiny TV, split-screen in their barracks. So that was really cool that they could find a game that works for them."

Army of Two: The 40th Day also features a substantial multiplayer offering, which Hutchison claims will be a significant progression from the first game. The title will includes three core multiplayer modes - Death-match, Capture the Flag and objective-based Warzone - supporting up to 12 players. A fourth bonus mode will be available in advance for anyone pre-ordering the game, but everyone else will have to wait 30 days after release. Hutchison said that the mode will be a survival style challenge similar to Gears of War 2's Horde, in which teams of four must stave off wave after wave of enemies.

"Even in the multiplayer, which is a big deal for us this time around, we wanted players to be able to migrate from a co-op pair in the main campaign to multiplayer modes," he said. "So you can split-screen against up to ten other people, or again network with your buddy. Co-op focus is what it's all about."

In terms of the game's art style, Hutchison argues that the team took inspiration from the shaky camera atmosphere of J.J. Abrams monster epic Cloverfield, but disaster classics as Towering Inferno also proved fertile ground for research.

"We wanted the world around the player to feel unstable and a place you can't rely on so to players will be really pushed to stick close to their partner," he explained. "That was the biggest visual influences on us. We also had another joke we would tell early on in the game that we wanted to be as much Bruce Willis in Die Hard and as little Steven Seagal as possible. So hopefully we hit that."

Army of Two: The 40th Day is available on PS3, PSP and Xbox 360.