Sorry, I read everyday but don't log in much. Military examples found easily at wikipedia

https://medium.com/@vishnu.sain.m21/why-major-companies-are-switching-to-linux-the-strategic-advantages-ad1169afd801

https://en.wikipedia.org/wiki/List_of_Linux_adopters

In July 2001,[26] the White House started switching whitehouse.gov to an operating system based on Red Hat Linux and using the Apache HTTP Server.[27] The installation was completed in February 2009.[28][29] In October 2009, the White House servers adopted Drupal, an open-source content management system software distribution.[30][31]
In April 2006, the US Federal Aviation Administration announced that it had completed a migration to Red Hat Enterprise Linux in one third of the scheduled time and about US$15 million under budget. The switch saved a further US$15 million in datacenter operating costs.[32][33]
By 2007, the United States Department of Defense uses Linux - "the U.S. Army is the single largest installed base for Red Hat Linux"[34] and the US Navy nuclear submarine fleet runs on Linux,[35] including their sonar systems.[36]
By 2008, the US National Nuclear Security Administration operates the world's tenth fastest supercomputer, the IBM Roadrunner, which uses Red Hat Enterprise Linux along with Fedora as its operating systems.[37]
In June 2012, the US Navy signed a US$27,883,883 contract with Raytheon to install Linux ground control software for its fleet of vertical take-off and landing (VTOL) Northrop Grumman MQ-8 Fire Scout drones. The contract involves Naval Air Station Patuxent River, Maryland, which has already spent US$5,175,075 in preparation for the Linux systems.[38]