An army marches on its stomach, but it plans those marches — and the subsequent PowerPoints about those marches — in Microsoft. The bureaucracy of war, the teeming thousands hunched over computers inside offices and campuses and an eponymous five-sided funhouse runs, like the rest of modernity, on software. This makes cybersecurity an everyday part of military work, and it means a big shift. Think about the Air Force’s migration to Windows 10 which is expected by the end of March.

An announcement about the news from the Air Force’s Public Affairs department reads like ad copy:

“In cyberspace … every Airman is an operator. They need to be aware of cyber threat and do their part by being prepared for the Win10 migration,” said Bill Marion, deputy chief, information dominance and deputy chief information officer. “The long term benefit is not just about defending the Air Force against the cyber threat, but having a reliable and capable computer to accomplish the mission.”

And if there’s a sense of urgency in the announcement, it might come from the fact that the original switch to Windows 10 was planned in 2015, with the expectation that deployment through all combatant commands, services, agencies and field activities would be complete by January 2017.

Whoops.

This is hardly the first time the Pentagon’s adopted a new operating system, only to find it still in use well after the mandated switch-over date. To get a glimpse of how the later-stages of the Windows 10 switch-over will play out, we need only go to the recent past, when the Pentagon tried to phase Windows XP, an operating system released in 2001.

Federal officials have known for more than six years that Microsoft will withdraw its free support for Windows XP on April 8, 2014. Despite a recent rush to complete upgrades, an estimated 10 percent of government computers — out of several million — will still be running the operating system on that date, company officials said.

That includes thousands of computers on classified military and diplomatic networks, U.S. officials said. Such networks have stronger defenses generally but hold more sensitive material, raising the stakes for breaches if they occur.

Even after the deadline, the Department of Defense continued to order support for Windows XP, with the Navy purchasing more XP support well into 2016, a year after the switchover to WIndows 10 was announced.

Invariably, the computers running pre-Windows 10 operating systems will dwindle, and the switchover will proceed to a satisfactory degree of accomplishment, even among the holdovers. But I wouldn’t expect older system to die entirely, and whenever the Pentagon switches to the post-10 operating system, expect to see Windows 10 kicking around on backwater computers. After all, this is the same Pentagon that still operates Windows 3.1.