China's economic rise has entailed increasing repercussions of its development in the rest of the world, including Latin America and the Caribbean. No wonder there is so much attention and hope for smoothness in China's current economic rebalancing. After all, what happens in China does not stay in China.
Foreign policy should reflect international realities, which change over time. The Republic of Korea was vulnerable to renewed North Korean aggression at the conclusion of the Korean War in 1953. Today, Seoul could do whatever was necessary to deter and defeat the Democratic People's Republic of Korea. There's no need for America to defend that which could be defended otherwise.
The TPP negotiations took place in a secret process dominated by the giant multinational corporations, and the final agreement is waiting to be approved or rejected by Congress - probably during the "lame duck" session, because that is when members are least likely to be held accountable for their votes.