Relations with South Korea have been far more straightforward. Seoul has proved a willing helper on U.S. foreign policy priorities such as Afghanistan and fighting climate change.
The allies have moved in lockstep in their diplomacy toward North Korea, which was accused of launching two military attacks in 2010 that sank a South Korean submarine and killed 50 South Koreans, almost sparking another war on the divided Korean Peninsula.
Obama and Lee have refused to offer fresh aid and incentives to North Korea without Pyongyang taking concrete action to show it is sincere about eventually giving up its nuclear weapons.
That policy of "strategic patience" and reluctance to jump back into negotiations has come in for criticism. While multinational disarmament talks have been suspended, North Korea has unveiled a uranium program that gives it a new means of generating fissile material for atomic bombs.
In recent months, however, both Seoul and Washington have held exploratory talks with Pyongyang, helping dial down tensions.
The United States is expected to hold another meeting with North Korea soon, to discuss how the six-nation disarmament-for-aid negotiations can get back on track. Although it is thought very unlikely Pyongyang would ever give up its nuclear weapons, talks are seen as a way of forestalling fresh aggression by the North.
Both South Korea and the United States are entering an election year and will want to avoid the kind of security crisis that could ensue following a nuclear test or military attack.