Though dwarfed by the U.S.' military, North Korea's is still one of the largest in the world, with 1.2 million active service members, nuclear weapons, and tens of thousand of artillery at its disposal.
According to two U.S. military sources which spoke to NBC, U.S. intelligence has identified at least two dozen targets in the DPRK, which they say consist of missile launch sites, testing grounds, storage facilities and support equipment.
North Koreans holding national flags march during a parade to mark the 60th anniversary of the signing of a truce in the 1950-1953 Korean War at Kim Il-sung Square, in Pyongyang July 27, 2013.
Mr Mattis issued a statement as he travelled to the West Coast.
Malcolm Turnbull, the prime minister of U.S. ally Australia, said he had discussed North Korea and recent United Nations sanctions imposed on Pyongyang with U.S. Vice President Mike Pence in a telephone call on Thursday.
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Not Donald Trump , not Barack Obama- it's been going on for a long, long time. "Our message is plain and simple". You are not wanted in this great commonwealth.
"My portfolio, my mission, my responsibility is to have military options if you need it", Mattis told reporters at a Defense Innovation Unit Experimental (DIUx) event in Mountain View, California, in his first remarks since North Korea threatened to launch missiles near Guam.
Secretary of State Rex Tillerson has urged calm, saying he sees no "imminent threat" of attack.
Former State Department official Joel S. Wit, the founder of 38 North, a website that analyzes North Korea, told Politico that Trump's claims risked undermining diplomatic efforts. He said that while the U.S. is pursuing diplomatic solutions, the combined military power of America and its allies is the most robust on Earth. On Thursday, Mattis went on to say the Trump administration is "prioritizing the threat" but also looking at "how to deal with it in a diplomatically effective manner".
'The current situation on the Korean Peninsula is highly complicated and sensitive. "So the American people should sleep well at night".