The US Department of Defense will upgrade 4 million devices to Microsoft's Windows 10 operating system, the technology company has revealed, in its biggest Win10 deal yet.
It marks the largest rollout so far for the seven-month old operating system, which Microsoft last month said was running on 200 million devices.
The Defense department is planning to have upgraded all its devices - including laptops, desktops and Surface tablets - within a year, an "unprecedented move" for an organisation the size of the department, Microsoft said.
The deal continues Defense's long relationship with Microsoft software.
The department spends around US$38 billion annually on cybersecurity and IT, Microsoft said, to help protect networks that are "getting shot at" nearly every day.
"Because the US Department of Defense is a prime target of cyber criminals and one of the largest and most complex organisations in the world, its leaders know the importance of securing its baseline systems," Microsoft said.
According to a department memo published by Microsoft, Defense first informed staff of its plans to move to the OS last November. It expects the move to be completed by January 2017.
"It is important for the department to rapidly transition to Microsoft Windows 10 to in order to improve our cybersecurity posture, lower the cost of IT, and streamline the IT operating environment," the memo reads.
Windows 10 has been certified by the US national information assurance program for use in national security systems. Microsoft's Surface tablet convertible has been certified for use within Defense.