This week North Korea has threatened to launch a preemptive nuclear strike on the US. They have also said that they wanted to end the armistice between them and South Korea. This was all a result of UN sanctions and US training drills in South Korea. The UN sanctions had said that they wanted North Korea to stop their nuclear arms program. The US drills in South Korea are annual and are used to train some of the South Korean military for possible scenarios of conflict with North Korea. video link: Korea threat
I do not think North Korea will carry out with the nuclear strike they are saying they will launch. They have made many threats like this before. They also have a large military and air force but they are both poorly trained and not well maintained.