China and North Korea

I just thought it was interesting how in America we tend to always focus on the relationships we have with other countries. Perhaps it was my lack of knowledge, but I had always thought that the United States had the most influence over North Korea. However, this overview of the North Korea and China article clearly shows that China has a lot invested in North Korea. I had always thought that North Korea was very much a hermit kingdom in the sense that it only exported and did not import. I find it interesting how China is basically the money behind North Korea, yet North Korea can still get China do follow their terms when it comes to things such as defectors. I was just curious as to why China doesn't take more of a powerful/demanding role in its relationship with North Korea.