From a former Microsoft employee, this is a very good read on the Microsoft-Nokia marriage... 

I suspect Ballmer and the board had their finger on the “acquire” button all along, mainly to ward off a competitive acquisition or to save Nokia from drowning and leaving Microsoft with no one to throw a Windows Phone-shaped life preserver to. A year ago, saving Nokia may have seemed less crucial, had it come to that. After all, Microsoft could just build its own hardware and forget the whole software licensing side of things altogether if it needed to (unlike the PC market and big Windows, they really had nothing to lose by doing this). I don’t think it was plan A. Maybe B or C. But in light of the Surface write-down, I think that contingency suddenly seemed a whole lot more risky, and that finger over the acquire button got twitchy.

Full article: 

http://brandonlive.com/2013/09/04/did-the-surface-write-off-cause-microsoft-to-buy-nokia/