The United States of America, the Republic of Korea, and Japan held a productive and substantive security meeting from April 16-17 in Washington, District of Columbia, to enhance trilateral defense cooperation in light of the evolving security environment in the region.... The officials reaffirmed that their governments will not accept North Korea as a nuclear-armed state, and agreed to closely coordinate to deter North Korean provocations.... In addition, the three countries held a productive discussion on cooperative measures for non-traditional security issues including humanitarian assistance and disaster relief, counter-piracy operations, and counter-ISIL efforts.[1]
Three years ago it was impossible to imagine the US and Iran co-operating about anything. Now the US has provided de facto aircover for Iranian operations against ISIS.
Koreans have not even allowed the Japanese to help them with the Sewol ferry even if they were within the reach.