The board told Ballmer they wanted him to stay, he says, and they did eventually agree to a slightly different version of the deal. In September, Microsoft announced it was buying Nokia’s devices-and-services business for $7.2 billion. Why? The board finally realized the downside: without Nokia, Microsoft was effectively done in the smartphone business. But, for Ballmer, the damage was done, in more ways than one. He now says it became clear to him that despite the lack of a new C.E.O. he couldn’t stay. Cultural change, he decided, required a change at the top, and, he says,“there was too much water under the bridge with this board.” The feeling was mutual. As a source close to Microsoft says, no one, including Gates, tried to stop him from quitting.