Steve Ballmer's smartphone strategy when he was head of Microsoft caused some strain between himself and company co-founder Bill Gates, according to a new interview with the former CEO. Ballmer revealed to Bloomberg that the two had a difference of opinion on the strategic direction of the company, specifically over how important it was to be in the hardware business.

From Bloomberg:

"There was a fundamental disagreement about how important it was to be in the hardware business," Ballmer said. "I had pushed Surface. The board had been a little -- little reluctant in supporting it. And then things came to a climax around what to do about the phone business."

Ballmer is of course referring to his decision to push the purchase of Nokia's phone business in 2013 for more than $9 billion — a purchase that would ultimately be written down after current CEO Satya Nadella took over. Ultimately, however, Ballmer regrets not getting into the hardware business sooner with respect to the mobile sphere:

"I would have moved into the hardware business faster and recognized that what we had in the PC, where there was a separation of chips, systems and software, wasn't largely gonna reproduce itself in the mobile world," he said.

Ballmer's comments reflect the feelings of current CEO Nadella, who recently acknowledged that Microsoft had "clearly missed the Mobile Phone." Nadella, however, is now focused on Microsoft's efforts to "grow new categories" — something which can be seen in the company's renewed focus on services and even the Surface team's emphasis on creating new categories with each device.