Mattis did not elaborate on the diplomatic results he referenced, but the UN Security Council at the weekend passed a new set of sanctions against Pyongyang over its weapons program, including bans on the export of coal, iron and iron ore, lead and lead ore as well as fish and seafood.
On Tuesday, Trump threatened North Korea with "fire and fury" if Pyongyang made any further threats against the United States.
The announcement Thursday warned that the North is preparing a plan to fire four of its Hwasong-12 missiles over Japan and into waters around the tiny island, which hosts 7,000 US military personnel on two main bases and has a population of 160,000. The president met this evening with Secretary of State Rex Tillerson and U.N. Ambassador Nikki Haley to discuss their diplomatic efforts.
"The tragedy of war is well enough known", Mattis said, responding to a question about the human toll of a nuclear confrontation between the DPRK and the U.S.
Former California Gov. Arnold Schwarzenegger told The Chronicle in an interview that he was confident that there are experts giving Trump advice on the volatile North Korea situation, even if the president has been bellicose in his Twitter attacks on the rogue nation.
The North Korea threat has been a foreign policy priority for the Trump administration since it took office in January, but it has taken center stage since Pyongyang test fired two intercontinental ballistic missiles last month.
But it also said that if the US and South Korea took a first strike against North Korea, China should intervene.
South Korea's military says North Korea will face a "stern and strong" response from Washington and Seoul if it acts on threats to fire missiles near Guam. "We should also make clear that, if the USA and South Korea carry out military attacks to overthrow the North Korean regime and change the political landscape on the Korean Peninsula, China will be determined to take actions to prevent them from doing so". He also held out hope that the tensions won't reach that point.