He has declared Russia to be the number one threat to the US.
He has threatened intervention in China's territorial affairs.
I was wrong. I thought Gen. Mattis was a reasonable choice as he rejects the efficacy of torture, and, according to Trump, convinced Trump that "torture doesn't work." Apparently Mattis cannot reach beyond this realization to higher geo-political realizations. Trump needs to fire Mattis who has placed the Pentagon in the way of normal relations with Russia.
There is no evidence in the behavior of Iran, Russia, and China to support Gen.Mattis' views. His definition of threat is the neoconservative one—a country capable of resisting US hegemony. This is a convenient threat for the military/security complex as it justifies an unlimited budget in order to prevail over such "threats." It is this hegemonic impulse that is the source of terrorism.