"We are going to appoint 'Mad Dog' Mattis as our secretary of defence. They say he's the closest thing to General George Patton that we have."
The Republican-controlled Congress will need to approve legislation bypassing the requirement so he can take up the role.
But at least one Democratic senator, Kirsten Gillibrand of New York, says she will not approve a waiver.
"Civilian control of our military is a fundamental principle of American democracy," Ms Gillibrand said in a statement, "and I will not vote for an exception to this rule."
http://www.bbc.co.uk/news/world-us-canada-38178625