After an illustrious career spanning four decades, U.S. Air Force Lt. Gen. Marc Sasseville has announced his retirement from the military. In an exclusive interview with ABC News, Sasseville shared his memories of a mission that could have been his last on September 11, 2001, when he took to the skies to protect the nation from further attacks.

On that fateful day, Sasseville received orders to prevent United Airlines Flight 93, hijacked by four al-Qaeda terrorists, from reaching its intended target in Washington, D.C. Alongside fellow F-16 pilot, Heather Penney, Sasseville took off from Joint Base Andrews in complete uncertainty of their mission's details.

The memories of the Pentagon engulfed in flames, with the acrid smell of burning concrete and jet fuel, remain etched in Sasseville's mind. He described the events of 9/11 as a "motivating imperative" to look forward and be prepared for future challenges.

As the orders came in to locate Flight 93, Sasseville and Penney made a harrowing decision. They would be willing to sacrifice their own lives by ramming their fighter jets into the hijacked plane if necessary. Sasseville was prepared to target the front of the jetliner, while Penney would target the tail section.

Behind the scenes, Sasseville kept his mission secret from his wife and two young children, aged five and three. His dedication to protecting others was exemplified by his choice to lead the mission himself, unwilling to ask anyone else to make the same sacrifice.

However, the heroic passengers and crew on Flight 93 fought back against the terrorists, preventing further tragedy. Sasseville expressed his admiration for their bravery, acknowledging that their actions saved him and his family from a very different outcome.

After landing to refuel, Sasseville and Penney were unknowingly chosen to escort Air Force One as President George W. Bush returned to Washington. Photographs captured the historic moment, with Sasseville's F-16 flying alongside the presidential jet.

Upon returning home, Sasseville kept the details of his role on 9/11 hidden from his loved ones, only revealing the truth later on. He retired as a three-star general and the number-two officer in the National Guard.

Lt. Gen. Sasseville's final flight on an F-16 took place on May 15, departing from Joint Base Andrews – the same base where he embarked on his pivotal mission almost two decades ago. The unarmed F-16 he flew on that day has since been modified by the Air Force to serve as a target for training purposes.

During his retirement ceremony, Sasseville expressed gratitude for the opportunity to serve and make a difference. He looks forward to spending time with his family, embarking on a well-deserved trip to Puerto Rico, and rediscovering a sense of normalcy.

As Lt. Gen. Sasseville concludes his military service, his unparalleled courage and commitment to protecting the nation will forever be remembered. His legacy serves as a reminder of the sacrifices made by those who stepped forward in times of crisis.