When the heads of Infinity Ward, the game development studio that created the hit Call of Duty series, were fired last March and most of their top talent quit soon after, publisher Activision Blizzard, said it didn't expect the turmoil to affect operations.

However, Infinity Ward's operations apparently have been affected enough that it has called in reinforcements to make the next Call of Duty military game. The studio was able to pull off prior games without aid, as it did with 2008's Call of Duty: Modern Warfare 2 and 2006's Call of Duty: Modern Warfare.

Activision has quietly tapped Sledgehammer Games, a Northern California studio, to help out with the making of Call of Duty: Modern Warfare 3, according to people familiar with the situation. The game has not been officially announced by Activision. But the title is well into production and is expected to come out in November, the same month that the publisher has annually released a new Call of Duty installment, people with knowledge of situation said. Modern Warfare 3 will be the first game Infinity Ward has produced since the departure of of company co-founders Jason West and Vince Zampella.

People close to the matter said Infinity Ward is working closely with Sledgehammer, which is led by a team recruited away from Activision's rival Electronic Arts Inc. The two studios are equally sharing duties of producing the single-player portion of the game, the sources said.

Activision has also brought in Wisconsin-based Raven Software, which is working on the online multi-player elements for Modern Warfare 3, sources said.

The decision to seek outside help is a departure for Activision, which in the past has left Infinity Ward alone to develop the Call of Duty: Modern Warfare games. The approach paid off, as the last two Modern Warfare games sold 13 million and 20 million units, respectively.

But pulling in the Sledgehammer team to help out on Modern Warfare 3 is creating a domino effect by forcing Activision to delay the start of production on a new Call of Duty spin-off game that the studio had begun working on before the upheaval at Infinity Ward, the people said.

This isn't just war games, as it were. Producing Call of Duty games on an annual schedule is critical to Activision. It's one of two franchises -- along with World of Warcraft -- upon which the video game giant is dependent for most of its revenue. So missing a release date or rushing out a game before it's ready could upset the company's projected revenue.

Call of Duty: Black Ops, produced by Activision studio Treyarch in 2010, was the best-selling video game in the U.S. last year, as was Modern Warfare 2 in 2009.

A spokesman for Activision declined to comment.

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-- Ben Fritz

Image: A scene from Call of Duty: Black Ops.