During that time, he addressed a variety of legal issues dealing with subjects such as international criminal law, counter-narcotics, interdictions, piracy, counterterrorism, weapons of mass destruction, non-proliferation, missile defense, foreign assistance, and treaty implementation.

While in the Marine Corps, he served as the first acting chief prosecutor of the office of military commissions and developed policies and processes for the tribunals envisioned to try those who planned the September 11 attacks in 2001.

The UK Supreme Court ruled that his transfer was in violation of international law (see Secretary of State for Foreign and Commonwealth Affairs v Rahmatullah).

On July 14, 2011, US forces accidentally released a secret form used to classify whether captives were an "Enduring Security Threat" to the American Civil Liberties Union.

[6] He was offered a more senior policy position in the Pentagon that would have allowed him to retain authority over detention issues generally, but he could not commit to staying in any new job long enough.

After the terrorist attacks of September 11, 2001, when he was a uniformed lawyer for the Marine Corps, he served as an adviser in the creation of the first version of President George W. Bush's system of military commissions trials.

In the Obama administration, he has been the primary official shaping policies for "law of war" detention at the prison at Guantánamo Bay and Bagram Air Base in Afghanistan.

On December 18, 2023, the Department of Defense Office of the Inspector General (DoD OIG) released a report as a result of a two-year investigation into Lietzau.