James Mattis is a retired United States Marine Corps general who served as the 26th Secretary of Defense from January 2017 to January 2019. Known for his strategic insight and leadership, he played a significant role in military operations during his career, including the Iraq War and the Afghanistan War. Mattis is often referred to by the nickname "Mad Dog" due to his tough demeanor and straightforward communication style. He is also an author and has written about military strategy and leadership, contributing to discussions on national security and defense policy in the United States.