Microsoft's cloud-computing work started in earnest in 2006, the same year Amazon launched Amazon Web Services.

Microsoft did have a model for what would work.

In 2008, Server & Tools chief Muglia and Andy Jassy, the leader of Amazon Web Services (AWS), struck a deal.

Nadella took over Server & Tools in 2011 with a mandate to push harder on cloud computing, colleagues say.

It didn't matter whether customers were ready for web-based tools, the thinking went. The industry was going that direction, and Microsoft could try to lead or risk getting left behind.

"Very quickly, the world changed," said Khalidi, who leads Azure's networking teams today.

Nadella placed Scott Guthrie, a career Microsoft engineering manager, in charge of the Azure team.

One of Guthrie's first moves was to gather Azure's leaders at an off-campus retreat for an experiment: He asked them to try to build an application using their own cloud service.

It didn't go well. Some features didn't work, and some managers failed to complete the online sign-up process to use Azure. "We didn't have everything quite right," Guthrie said.

The team rewrote Azure, with a focus on ease of use and the sort of tools Amazon was having success with. It also had to unlearn lessons from decades spent selling out-of-the-box software.

In the old world, how a product performed was a customer-service issue, walled off from engineers working on adding new features.

In the world of live web services, customer service became part of the engineering work.

Nadella instituted weekly meetings in which senior leaders spent hours delving into how customers used Microsoft services, as measured by about 3,000 metrics.

Azure relaunched in 2013 with a set of infrastructure services that performed many of the same functions as Amazon's AWS.

A month after Nadella replaced Ballmer as CEO in 2014, he signed off on a cosmetic, but hugely symbolic change. "Windows Azure" was renamed "Microsoft Azure."

Windows had been the center of Microsoft's universe for decades. Products had to play well with the operating system or else they wouldn't last long.

Dropping " Windows" was strategic, a reminder to potential customers that Azure could run more than Windows. It was also symbolic of the cultural shift Nadella was trying to engineer as Microsoft, formerly hostile to software built outside its walls, had decided to play nice with Azure.