North Korea has been a constant source of anxiety to South Korea, Japan and the US. US relations are not all that great with Japan and South Korea. It is more of a tentative and symbiotic relationship where both countries benefit from the support that the US can provide.
When it comes to the military, South Korea and Japan would prefer the US military to move out of their countries to a base close but US owned, such as in Guam. The help is still there, but the military moves would not be on their soil anymore. Since this won’t happen it is best to at least have the US around in case North Korea decides that it wants start creating trouble again.
The new and younger leader is trying to promote an army marshal. He just kicked the top general out of the military, which was considered a bid to establish his authority on the military. This move was made just seven months after he gained power. Kim Jong-un is probably not going to change much that his grandfather and father left behind in terms of policies. These policies are keeping most North Koreans starving and kept the country isolated from the rest of the world. This makes it difficult for the citizens, since they are unable to move comfortably.
The new marshal will be Hyon Yong-chol. He is little known to many, even in South Korea. It was stated that he is in his 60s and became an important individual in the army during 2007. In 2010 he was described as a leader who would help Kim Jong-un.