In a meeting between outgoing President Obama and current President Trump in 2016, Obama warned about the growing threat of North Korea's nuclear weapon program.  Years of 'strategic patience' had not paid off and North Korean nuclear technology was rapidly advancing , and it would soon become Trump's first foreign policy crisis. On the first of January 2017, North Korean leader Kim Jong-un stated in his annual New Year's address that North Korea had 'entered the final stage of preparation for the test launch of intercontinental ballistic missile'.  This would be the start of heightening tensions, and the U.S. would soon replace the 'strategic patience' strategy with a more aggressive stance towards North Korea.  The next few months were filled with threats from both sides and North Korean nuclear/missile tests, with fears about a possible war growing. 
This changed at the end of 2017, after North Korea tested a new ICBM. Kim Jong-un announced that the test “meets the goal of the completion of the rocket weaponry system development set by the DPRK [Democratic Peoples’ Republic of Korea]”.  It was a clear shift in policy that was further emphasized in Kim Jong-un's 2018 New Year's Address. He re-iterated that North Korea is a 'responsible, peace-loving nuclear power' and that it 'will develop good-neighbourly and friendly relations with all the countries that respect our national sovereignty and are friendly to us'.  He also stated his desire for North Korea to participate in the Winter Olympics that would be held that year in South Korea. Talks between both Koreas were started and provided an opening for broader talks to reduce tensions.  The South Koreans also involved the U.S. and it eventually culminated in a historic summit between U.S. President Donald Trump and North Korean leader Kim Jong-un in Singapore, at which they both signed a statement.  A key sentence in the Singapore statement is the 'denuclearization of the Korean Peninsula', yet its meaning is highly contested to this day.  While the U.S. sees it as North Korea giving up their nuclear weapons, North Korea views it as clearing the whole Korean peninsula of nuclear weapons, including the nuclear umbrella that the U.S. has over South Korea. And while tensions dropped significantly and nuclear/missile tests stopped (although production continued ), the core issue remains.
But tensions are currently rising once again, the Hanoi summit earlier this year failed due to U.S. unrealistic position: sanctions on North Korea would only be lifted after it would give up all of its nuclear weapons.  North Korea wants more gradual sanction relief, which is more realistic.  The failed summit froze talks between the countries, despite a promise they would be restarted when Trump and Kim met at the Demilitarized Zone (DMZ) between both Koreas in June.  U.S.-South Korea military exercises in August further strained their relationship, with North Korea conducting multiple missile tests in the span of a few weeks to signal its opposition to the exercises.  In his New Year's Address for this year, Kim Jong-un has already threatened to 'consider a new path if U.S. doesn’t keep its promises'.  What this new path will look like is unclear, but the U.S. will need to recalibrate its policy to be more realistic  if it wants to break the deadlock and prevent the situation from further heating up.