Inside the Pentagon, Mattis was known for being assertive with the use of U.S. forces, said Bryan Clark, who was a top aide to Adm. Jon Greenert, the chief of naval operations from 2011 to 2015. Mattis leaned on the Navy to keep two aircraft carriers in the Persian Gulf as a means to counter Iran, creating some strain on the service's budget and resources.
“He pushed a lot more of a hawkish tone towards Iran” said Clark, now an analyst with the Center for Strategic and Budgetary Assessments. “At the time, the Obama administration was trying to use the ‘carrot and stick’ method to get Iran to the table: The carrot being the nuclear negotiations and the sticks being the stepped up carrier presence. Mattis favored using the stick until the adversary cedes to your wishes — then bring out the carrots.”