When Microsoft CEO Satya Nadella takes the stage on Wednesday at Microsoft Build, the most important event of the year for the once mighty software maker, it represents something of a last best chance to win over mobile developers.

For all its power in the PC era, Microsoft has struggled to convince developers to create apps for its Windows Phone system, which has badly lagged rivals Google and Apple. Failing to win support at its annual developers conference this week could be fatal to its phone business.

Nadella and senior executives will tout the benefits of the latest Windows 10, which will unite its mobile and PC operating systems and allow developers the ability to write a program that can run on phones, tablets, PCs and even the Xbox and HoloLens. Today, programmers need to write different versions for the platforms.

Unlike previous big operating system upgrades, Microsoft will give this one away for free in the hope that more customers will upgrade. Typically, most customers wait to buy a new PC before upgrading their operating systems.

And here’s why bringing together PC and phone software development is so important: On the phone side, the company’s market share remains fairly tiny. Last quarter, Microsoft said it sold 8.6 million Lumia phones, as compared to the 61 million iPhones Apple sold over the past three months. And far more Android phones were sold than that.

The PC market, sluggish as it may be, presents a vastly different picture. IDC forecasts that nearly 300 million PCs will be sold this year, a nearly 5 percent drop from last year, but still a big number. The majority of these will run Windows, so Microsoft has an opportunity to get significant numbers of computers quickly onto its new operating system with the free upgrade.

With the new Windows 10, existing Windows developers will, for the first time, have an opportunity to watch their work proliferate across the bigger Microsoft empire and across multiple devices. But sustainable success for Microsoft can only be gained if it can bring on new developers who have, until now, avoided both Windows Phone and the company’s new-style Windows applications.

In addition to getting those PCs out there, Microsoft has to get computer users spending more time running the new-style Windows programs — the kind that work across PCs and phones — rather than the old-style Windows applications. With Windows 10, the distinction between the two is less jarring, but Microsoft still needs to make consumers more aware both of the store that serves up the new programs and of the titles available.

Aiming to show that universal apps are the new normal, Microsoft has taken to calling them Windows Apps and calling the old kind Windows desktop apps.

At this week’s Build conference in San Francisco, Microsoft will again make its Windows 10 case, this time pushing hard at developers. It’s a case the company has been trying to establish for more than a year now, but with Windows 10 due out in just a few months, Microsoft needs to put on the full-court press. Microsoft will show its most detailed look yet at what the final version of the software will look like, though early preview versions have painted a pretty clear picture of software that merges the look of Windows 7 with the ability to run new-style Windows 8 apps side by side.

Some developers will certainly take a wait-and-see approach, but Microsoft needs to — at a minimum — grab their attention and ideally inspire at least some in the crowd to do a little early experimentation for the new operating system. Expect Microsoft to pledge a lot of money and promotion for those who do early interesting work on the new Windows.