He then attended the United States Military Academy at West Point, where he studied international relations and graduated near the top of his class in 1988.

As a newly commissioned second lieutenant, he was chosen as a Rhodes Scholar and studied international relations at St John's College, Oxford, earning his MPhil in 1990.

While teaching at West Point, he was affiliated with the Strategic Studies Institute at the United States Army War College[6] for which he co-authored a book on military professionalism in 1999.

A revised version of his dissertation was published in 2002 as Learning to Eat Soup with a Knife, titled after an observation made by T. E. Lawrence about the challenges of fighting guerrilla forces.

While he was military assistant to Wolfowitz and England, Nagl co-authored the new United States Army and Marine Corps Counterinsurgency field manual as part of a team overseen by Generals David Petraeus, the former director of the Central Intelligence Agency, and James N. Mattis, the former commander of United States Central Command.

[19] In January 2011, he and CNAS chief executive officer Nathaniel Fick wrote an op-ed in the New York Times supporting the Obama administration's troop surge and reporting on its early successes.

[20] In April 2011, he wrote favorably about President Obama decision to name Leon Panetta, then director of the CIA, as Secretary of Defense, and General David Petraeus, then commander of the International Security Assistance Force in Afghanistan, with whom Nagl had worked on the new Army/Marine Corps counterinsurgency manual, as director of the CIA, noting the increasingly intertwined nature of the relationship between the intelligence and defense establishments in the United States.

[26] Dr. Nagl is an Advisory Board Member of Spirit of America, a 501(c)(3) organization that supports the safety and success of Americans serving abroad and the local people and partners they seek to help.

[30] In 2020, Nagl published an op-ed encouraging military action if President Donald Trump refused leaving office at the end of his constitutional term.