North Korea prides itself on being the shrimp which rules the whale through Juche (political independence) and bellicose bluster backed by a large standing army along with an aggressive nuclear program.
Juche is somewhat illusory as Pyongyang is dependent on trade with China and uses its weapons programs (both nuclear and missile programs) as a means to threaten and profit. Despite there just being an armistice which ended the Korean War in 1953, there has been a trend since the 1994 Clinton-Kim "Agreed Framework" of non-nuclear proliferation to diplomatically buy peace with Pyongyang, all the while while the Kim regimes continued nuclear testing, ballistic missile tests and close contacts with Iran, Syria and potentially non-state terror groups.
After President Trump's United Nations Ambassador Nikki Haley achieved an unanimous 15-0 Security Council vote (including the Russian Federation and the Peoples' Republic of China) for sweeping e economic sanctions against the Hermit Kingdom, North Korea warned that there would be a thousand fold revenge upon the United States. Moreover, there have been reports in the West that North Korea has advanced in miniaturizing nuclear warheads that could hit the continental United States.
In this explosive environment, President Donald Trump warned: "North Korea best not make any more threats to the United States. They will be met with fire and fury like the world has never seen."
Some might consider this returning bluster for bluster. But the United States has three aircraft carriers in the region. Moreover, Spirit B-2 aircraft (which can carry nuclear bombs) have overflown the Korean Peninsula. Furthermore, considering how many former Marines are in key positions in the Trump Administration (Sec Def Gen. James Mattis, Joint Chiefs Chair Joe Dunford, NSC Advisor Gen. H.R. McMaster and Chief of Staff John Kelly) and the deference which President Trump gives to the military, there is little doubt that he will act if they determine that North Korea is an existential threat to the United States.