Mattis' commands saw him take direct charge over a number of different operations:
As NATO's Supreme Allied Commander for Transformation, he was tasked with implementing new standards, tactics, and concepts into the Alliance's militaries. He had similar tasks as commander of US Joint Forces Command — though only for the US Military's five branches.
As Commander of United States Central Command (USCC), he oversaw operations in Iraq and Afghanistan, as well as parts of Syria, Iran, and Yemen.
The Obama White House apparently did not place much trust in Mattis, believing him to be too hawkish, especially on Iran. Nevertheless, he served as Commander of USCC until his retirement in 2013.