COLORADO SPRINGS, Colo. -- The Soldiers of the 100th Missile Defense Brigade are the nation's ground-based mid-course missile defense experts, and have provided America's safeguard from intercontinental ballistic missile attack since 2003.But the unit was born of modest beginnings.For many years, the United States recognized the need for a dynamic ballistic missile defense system. However, the idea of intercepting an enemy warhead in space -- a practice many have described as hitting a bullet with a bullet -- pre-dated the technology by decades.Following the attacks of Sept. 11, 2001, the George W. Bush administration withdrew the United States from the Anti-Ballistic Missile Treaty. Later in 2002, the White House issued National Security Presidential Directive 23, ordering the deployment of an initial limited homeland defense capability by 2004.President Ronald Reagan's Strategic Defense Initiative vision was realized and the concept of a defense system to defeat enemy missiles outside of the earth's atmosphere became a reality.On Oct. 16, 2003, the Department of Defense activated the 100th Missile Defense Brigade, then known as the Colorado Army National Guard Missile Defense Brigade, under the flag of U.S. Army Space and Missile Defense Command/Army Forces Strategic Command."It began as a collection of people who really didn't know much about missile defense," said Brig. Gen. Gregory Bowen, deputy director, Global Operations (DJ3), Global Operations Directorate, United States Strategic Command and former brigade commander. "We were brought in from a number of different places and formed into a unit that became the 100th Brigade."Command Sgt. Maj. Russell Hamilton has served in many roles with the 100th Missile Defense Brigade, including as the brigade's command sergeant major from 2010 to 2014, and said the unit's early days were exciting."There was so much mystery, so much unknown about this thing that people called GMD," said Hamilton, who now serves as the Colorado Army National Guard command sergeant major. "Nobody knew what it was. Everybody had their visions of the movie 'War Games,' or 'Star Wars,' or something like that."Although the brigade was activated, it took several months to become operational. Fort Greely, located in the heart of the Alaska interior, was selected as the tactical site for the GMD system. Soldiers were needed to operate the system and guard the Missile Defense Complex there, and so the 49th Missile Defense Battalion of the Alaska National Guard stood up in January 2004.Bowen said that much has changed at Fort Greely since then."Fort Greely was an interesting place to begin with as it had been closed due to Base Realignment and Closure," said Bowen, who served as the original commander of the 49th Missile Defense Battalion, 2003-2006, and later as the commander of the 100th Missile Defense Brigade, 2009-2012. "They brought it out of BRAC for missile defense. When I arrived in the spring of 2003, what is now the Missile Defense Complex was just a huge construction site. Fort Greely, the garrison itself, was nothing like it is today."As Fort Greely was restored and rebuilt, ground-based interceptors -- the bullets of the GMD system -- were placed there.By the summer of 2006, the system was armed and ready. The timing was apt, as North Korea launched a series of test missiles in July 2006. One such missile, the Taepodong-2, featured an estimated range of nearly 6,000 miles.Although it failed shortly after launch, that test, and a North Korean underground nuclear test that October, set the tone for brigade operations for years to come."The training got more rigorous," Hamilton said. "There was a lot that wasn't known about the system or the program as we first came in. So a lot of us, both on the instructor side, on the evaluator side and the operators themselves, were all learning together as we went through some of these things."24/7/365 operations became the norm for the entire brigade, according to retired Col. Michael Yowell, who served as the second brigade commander, 2006-2009. As the unit matured and the Soldiers refined their tactics, techniques and procedures, the system was continuously developing and becoming more advanced. This included adding a small detachment of Soldiers and additional ground-based interceptors at Vandenberg Air Force Base, California."The brigade is ever-evolving," Yowell said. "Unlike when you have a normal weapons system that takes months and years to develop and test and prototype and finally field, we were fielding and updating software and equipment on a constant basis in concert with our partners with the Missile Defense Agency."The system and materiel in place today are the products of countless hours of research, development and evaluation. William Spriggs, a ground-based mid-course defense instructor with the USASMDC/ARSTRAT Future Warfare Center, said this includes several missile flight tests, which have demonstrated increasing success."This is a very complicated weapons system -- one of the most complicated things we've ever done as a nation is missile defense," Spriggs said. "You expect some failures. That's how science works. You fail, you fix it and you make it better."On May 30, 2017, the Soldiers of a 100th Missile Defense crew showcased the capabilities of the system, launching an interceptor from Vandenberg that collided with and destroyed its target -- launched from the Marshall Islands in the South Pacific, nearly 5,000 miles away -- in space.Flight Test Ground-based Interceptor 15, or simply FTG-15, was the first successful exoatmospheric intercept of an intercontinental ballistic missile-class target in human history.The current 100th Missile Defense Brigade commander, Col. Kevin Kick, said that the success of FTG-15 proved that the system works against the threat it was intended to fight.That threat was on display throughout 2017 as North Korea conducted another underground nuclear test and launched a number of long-range missile tests that were deemed capable of hitting the United States mainland.The term "ICBM" became globally recognized. Hamilton said that during this time of heightened tension, he wanted to be back alongside the Soldiers of the 100th Missile Defense Brigade."This is where you want to be when something like that is going on in the world," Hamilton said. "But knowing the Soldiers and leadership team that we have in the 100th, I had absolute confidence we'll be able to deal with whatever challenge came our way."The 100th Missile Defense Brigade is perhaps the most unique unit in the United States military because it is the only missile defense brigade in the Department of Defense, and because it mainly composed of National Guard Soldiers, who are deployed in place and fighting an enemy threat from abroad.Spriggs -- who was a member of the first crew to operate the GMD system -- said when he looks at his National Guard retirement statue at home, it makes him think of the 100th Missile Defense Brigade's Soldiers and mission."It's the Minuteman with a plow and a musket," said Spriggs, describing the statue. "I kind of look at GMD the same way. We're guarding the homeland. The threat has obviously changed since 1636, but the mission is still the same. We're just a bunch of folks with regular, normal lives that come in every day and stand watch, and I think it's appropriate that the National Guard does that, because we are defending the homeland."The Soldiers of the 100th Missile Defense Brigade have helped revolutionize homeland missile defense for the last 15 years. Whatever the future may hold, Hamilton said the 100th Missile Defense Brigade will be ready."Is it the likely course of action where we have to launch an interceptor? Probably not," Hamilton said. "But on America's worst day where something goes wrong in the world, and America is threatened, we've got to be ready. Adversaries only have to get it right one time to create something catastrophic. We have to get it right every single time."