President Barack Obama arrived in South Korea on Sunday for a three-day trip centered on an international nuclear security summit in Seoul.Nuclear non-proliferation is a major reason (but by no means the sole reason) for continued US military support of South Korea and Japan, two countries which have foregone their building their own nuclear arsenal in exchange for a robust alliance with the United States.
He flew into Seoul, where he is expected to hold a bilateral meeting with his South Korean counterpart Lee Myung-bak.
Top officials from 54 countries, including China and Russia, will attend the summit meeting on Monday and Tuesday.
But its message of international cooperation has been overshadowed by North Korea's announcement last week that it is planning to carry out a rocket-powered satellite launch in April.
South Korea has said it considers the satellite launch an attempt to develop a nuclear-armed missile, while the United States has warned the move would jeopardize a food-aid agreement reached with Pyongyang in early March.
President Lee has already said he will use the summit to drum up international support against the actions of his northern neighbor.
North Korea says it has a right to a peaceful space program and has invited international space experts and journalists to witness the launch.
Hemming in a virally expansionist China is another major reason, but even China doesn't want North Korea to get nukes because (a) it makes it more likely that historical adversary Japan will get them and (b) rogue nukes could end up in the hands of Muslim separatists along China's western frontier. Ditto with Russia on Pyongyang having nukes.