Here's how Marc Benioff tested Satya Nadella before deciding to cozy up with Microsoft

Salesforce CEO Marc Benioff has had a rocky relationship with Microsoft, to say the least. During Microsoft's Steve Ballmer-era, the two companies constantly bickered and trash-talked each other.

But the relationship has improved a lot in the nearly 16 months since Satya Nadella replaced Ballmer as Microsoft's CEO.

"We have an amazing relationship with Satya, starting from about a year ago," Benioff declared at a press event Tuesday that announced Salesforce's partnership with accounting software Sage.

So how did this cozy bromance get started?

Benioff attributes it to a dinner meeting he had with Nadella last year. Benioff recounted at the event that he had heard through his friend John Thompson, Microsoft's chairman, that there might be a chance to form a more collaborative relationship with Microsoft under Nadella.

But Benioff first wanted to test if Microsoft's newly appointed CEO was serious about working together.

So he asked Nadella if he could poach one of his key lieutenants, Randy Kern, to be Salesforce's new chief infrastructure officer.

Nadella, in his soft-spoken demeanor, looked up at Benioff, and simply said, "Yes, I will let you hire him."

Benioff was shocked. But he also realized right away that the door was open for more collaboration.

Soon after, in May, Salesforce and Microsoft announced a partnership deal that would allow the two companies to work more closely, including plans to allow Salesforce to run on Windows devices and Office 365. Salesforce has also launched products that integrate with Microsoft Outlook, SharePoint, and Excel, as well as its recent search technology Delve.

"We kind of have the old Microsoft back, which is a Microsoft open for businesses, ISVs, and software developers like to us to integrate," Benioff added. "We have an incredible partner with Microsoft."

In fact, Salesforce isn't the only bitter-rival-turned-partner for Microsoft under Nadella's leadership. Companies like Box and NetSuite, software providers that have historically had a bumpy relationship with Microsoft, have all formed new partnership deals one way or another, becoming friendly competitors, a thought that was hardly imaginable when Ballmer was at the helm.

"Steve Ballmer would never let us hire anybody from Microsoft," Benioff said at the Sage event.