Note: This article was cross-posted from Windows IT Pro because of its importance to my readers. --Paul

In a stunning and unexpected move, Microsoft announced Monday evening that president Steven Sinofsky, who had most recently lead the development of both Windows 8 and the Surface family of tablets, is leaving the company effective immediately. His most senior lieutenant, Julie Larson-Green, will lead all Windows software and hardware engineering going forward.

Microsoft CEO Steve Ballmer claimed Sinofsky’s departure was a “mutual” decision but hinted that Sinofsky didn’t work well with others and prevented Windows from moving forward more quickly. These are charges that have frequently been directed at the divisive Mr. Sinofsky, who has many detractors inside the company.

“I am grateful for the many years of work that Steven has contributed to the company,” Microsoft CEO Steve Ballmer said. “The products and services we have delivered to the market in the past few months mark the launch of a new era at Microsoft … To continue this success it is imperative that we continue to drive alignment across all Microsoft teams, and have more integrated and rapid development cycles for our offerings.”

Larson-Green isn’t the only one getting a promotion. Tami Reller will retain her roles as Chief Financial Officer and Chief Marketing Officer but will now assume responsibility for “the business of Windows,” as Microsoft put it. Both Larson-Green and Reller will now report directly to Mr. Ballmer.

Steve Ballmer and Steven Sinofsky both addressed the change in separate letters to employees, which I’ve published in their entirety on the SuperSite for Windows.

Windows Leadership Changes – The letter from Steve Ballmer

Windows Leadership Changes – The letter from Steven Sinofsky

In his letter, Mr. Sinofsky denounces any suggestion that he was ousted from the company or that the timing of this departure is related in any way to poor sales of Windows 8 or Surface. My gut feeling is that Sinofsky simply ostracized too many people in and out of Microsoft.

“I can assure you that none [of the speculation] could be true as this was a personal and private choice that in no way reflects any speculation or theories one might read—about me, opportunity, the company or its leadership,” he wrote. “This announcement is effective immediately and I will assist however needed with the transition.”

Regardless of this explanation, there is almost certainly more to this story than either Ballmer or Sinofsky is letting on, and it’s apparent that Mr. Sinofsky’s tendencies towards internecine warfare have finally caught up with him. Too many sources have told me stories about executives and employees leaving the Windows division, or the company entirely, because of Sinofsky’s tactics.

Once you accept that Sinofsky is gone, the only real surprise is that his close confidant Larson-Green has been left in charge or Windows engineering. The sources I’ve spoken with tonight claim she’s neither engineer nor product visionary, and wonder if this isn’t a temporary, transitional assignment.